Method for making multilayer optical films

ABSTRACT

Methods and apparatuses are provided for the manufacture of coextruded polymeric multilayer optical films. The multilayer optical films have an ordered arrangement of layers of two or more materials having particular layer thicknesses and a prescribed layer thickness gradient throughout the multilayer optical stack. The methods and apparatuses described allow improved control over individual layer thicknesses, layer thickness gradients, indices of refraction, interlayer adhesion, and surface characteristics of the optical films. The methods and apparatuses described are useful for making interference polarizers, mirrors, and colored films that are optically effective over diverse portions of the ultraviolet, visible, and infrared spectra.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.09/229,724, filed Jan. 13, 1999, now abandoned which is acontinuation-in-part of U.S. patent application Ser. No. 09/006,288filed on Jan. 13, 1998, now abandoned, and incorporated herein byreference.

FIELD OF THE INVENTION

The present invention relates to processes for making polymericmultilayered films, and in particular to coextruded multilayered opticalfilms having alternating polymeric layers with differing indices ofrefraction wherein at least one of the polymers is able to develop andmaintain a large birefringence when stretched.

BACKGROUND OF THE INVENTION

The present invention relates to processes for making polymericmultilayered films, and more particularly to coextruded multilayeredoptical films having alternating polymeric layers with differing indicesof refraction. Various process have been devised for making multilayerfilm structures that have an ordered arrangement of layers of variousmaterials having particular layer thicknesses. Exemplary of thesestructures are those which produce an optical or visual effect becauseof the interaction of contiguous layers of materials having differentrefractive indices and layer thicknesses.

Multilayer films have previously been made or suggested to be made bythe use of complex coextrusion feedblocks alone, see, e.g., U.S. Pat.Nos. 3,773,882 and 3,884,606 to Schrenk, and the suggestion has beenmade to modify such a device to permit individual layer thicknesscontrol as described in U.S. Pat. No. 3,687,589 to Schrenk. Suchmodified feedblocks could be used to make a multilayer film with adesired layer thickness gradient or distribution of layer thicknesses.These devices are very difficult and costly to manufacture, and arelimited in practical terms to making films of no more than about threehundred total layers. Moreover, these devices are complex to operate andnot easily changed over from the manufacture of one film construction toanother.

Multilayer films have also been made by a combination of a feedblock andone or more multipliers or interfacial surface generators in series, forexample as described in U.S. Pat. Nos. 3,565,985 and 3,759,647 toSchrenk et al. Such a combination of a feedblock and interfacial surfacegenerator is more generally applicable for producing a film having alarge number of layers because of the greater flexibility oradaptability and lesser manufacturing costs associated with afeedblock/ISG combination. An improved ISG for making multilayer filmshaving a prescribed layer thickness gradient in the thicknesses oflayers of one or more materials from one major surface of the film to anopposing surface was described in U.S. Pat. Nos. 5,094,788 and 5,094,793to Schrenk et al. Schrenk described a method and apparatus in which afirst stream of discrete, overlapping layers is divided into a pluralityof branch streams which are redirected or repositioned and individuallysymmetrically expanded and contracted, the resistance to flow and thusthe flow rates of each of the branch streams are independently adjusted,and the branch streams are recombined in an overlapping relationship toform a second stream which has a greater number of discrete, overlappinglayers distributed in the prescribed gradient. The second stream may besymmetrically expanded and contracted as well. Multilayer films made inthis way are generally extremely sensitive to thickness changes, and itis characteristic of such films to exhibit streaks and spots ofnonuniform color. Further, the reflectivity of such films is highlydependent on the angle of incidence of light impinging on the film.Films made with the materials and processes heretofore described aregenerally not practical for uses which require uniformity ofreflectivity.

Several of the patents and applications discussed above containteachings with respect to introducing layer thickness gradients intomultilayer polymeric bodies. For example, U.S. Pat. No. 3,711,176 toSchrenk et al., teaches that it is desirable that a gradient or otherdistribution in the thicknesses of layers of one or more materials beestablished through the thickness of the film. Methods for creatinggradients include embossing the film, selective cooling of the filmduring final stretching, and the use of a rotating die to create thelayers as described in U.S. Pat. Nos. 3,195,865; 3,182,965; and3,051,452. These techniques attempted to introduce layer thicknessgradients into an already extruded film, and did not permit precisegeneration or control of the gradients. U.S. Pat. No. 3,687,589 toSchrenk et al teaches the use of a rotating or reciprocating shearproducing means to vary the volume of material entering the feed slotsof a coextrusion feedblock where the polymer streams are subdivided.Precise control of volumetric flow rates using such a device isdifficult to achieve. In U.S. Pat. No. 5,094,788, Schrenk et al teachusing variable vanes in an ISG downstream from a coextrusion die tointroduce a layer thickness gradient into a multilayer polymer meltstream. U.S. Pat. No. 5,389,324 to Lewis et al describes control of therespective flow rates of the polymeric materials in the substreams toprovide a differential in the volume of material flowing through each ofthe substreams. Because of the differential in the volume of thepolymeric materials flowing in the substreams making up the compositestream, the individual layers in the body have a gradient in thethicknesses. The flow rate is controlled either by providing atemperature differential among at least some of the substreams, causingchanges in the viscosities of the polymeric materials and therebycontrolling their flow, or the flow rate is controlled by modifying thegeometry of the passages or feed slots through which the plastifiedpolymeric materials flow in the feedblock. In this way, the pathlengths, widths, or heights of the substreams can be modified to controlthe flow rate of the polymer streams and thus the thickness of thelayers formed.

To form a multilayered film, after exiting either a feedblock or acombined feedblock/ISG, a multilayered stream typically passes into anextrusion die which is constructed so that streamlined flow ismaintained and the extruded product forms a multilayered film in whicheach layer is generally parallel to the major surface of adjacentlayers. Such an extrusion device is described in U.S. Pat. No. 3,557,265to Chisholm et al. One problem associated with microlayer extrusiontechnology has been flow instabilities which can occur when two or morepolymers are simultaneously extruded through a die. Such instabilitiesmay cause waviness and distortions at the polymer layer interfaces, andin severe cases, the layers may become intermixed and lose theirseparate identities, termed layer breakup. The importance of uniformlayers, i.e., layers having no waviness, distortions, or intermixing, isparamount in applications where the optical properties of themultilayered article are used. Even modest instabilities in processing,resulting in layer breakup in as few as 1% of the layers, may severelydetract from the reflectivity or appearance of an article. To formhighly reflective bodies or films, the total number of layer interfacesmust be increased, and as the number of layers in such articles isincreased in the coextrusion apparatus, individual layer thicknessesbecome smaller so that the breakup of even a relatively few layers cancause substantial deterioration of the optical properties on thearticle. Problems of layer breakup are especially severe formultilayered bodies in which individual layer thicknesses approach about10 μm or less adjacent to the walls of the feedblock, multiplier, orextrusion die. Flow of multiple polymer layers through the feedblock andISG typically entails both shear and extensional flow, while flowoutside of the extrusion die is shear-free extensional flow. Layerbreakup occurs inside flow channels very close to the channel wallswhere shear flow predominates, and is affected by such factors as smalllayer thickness, shear stress, interfacial tension between polymerlayers, interfacial adhesion between the polymer melt and channel walls,and various combinations of these factors.

Several potential suggestions have been made to minimize flowinstability, including increasing skin layer thickness nearest the diewall, decreasing the viscosity of the layer nearest the die wall byeither increasing temperature or switching to a lower viscosity resin,reducing the total extrusion rate, or increasing the die gap. In U.S.Pat. No. 4,540,623 to Im et al, the use of sacrificial or integral skinlayers on the order of from about 1 to 10 mils (25.4 to 254 μm) isdescribed to ease processing and to protect the surfaces from damage.These exterior skin layers are added immediately prior to the multilayerfilm exiting from the forming die or prior to layer multiplication. InU.S. Pat. No. 5,269,995 to Ramanathan et al, the use of protectiveboundary layers (PBLs) of a heat plastified extrudable thermoplasticmaterial is taught to minimize layer instabilities. These layers may beinternal to the multilayer body and/or on the external surfaces andgenerally serve to prevent layer breakup during the formation andmanipulation of the multiple layers of polymers in a coextrudedmultilayer polymeric body.

While the previous discussion applies to multilayered films in general,often independent of the chemical, physical, or optical properties ofthe materials that make up the multilayered stack, by selective choiceof materials and proper control of subsequent processing steps,multilayered films with enhanced optical or physical properties can beobtained. For example, U.S. Pat. Nos. 5,486,949 and 5,612,820 to Schrenket al describe the use of birefringent polymers for the fabrication ofcoextruded polymeric multilayer optical films useful as interferencepolarizers. The birefringent polymers can be oriented by uniaxial orbiaxial stretching to orient the polymer on a molecular level such astaught in U.S. Pat. No. 4,525,413 to Rogers et al. in order to obtaindesired matches or mismatches of the in-plane refractive indices toreflect or transmit desired polarizations. Further, in U.S. patentapplication Ser. No. 08/402,041 to Jonza et al the use of birefringentmaterials useful for making interference polarizers and mirrors isdescribed in which control of the relationships between the in-plane andout-of-plane indices of refraction gives coextruded polymeric multilayeroptical films with improved optical properties at non-normal angles.

Recent developments in materials available for use in making polymericmultilayer optical films, and new uses for optical films which requireimproved control of layer thickness and/or the relationships between thein-plane and out-of-plane indices of refraction, have been identified.Processes described heretofore typically are not able to exploit thepotential of the new resins available and do not provide the requireddegree of versatility and control over absolute layer thickness, layerthickness gradients, indices of refraction, orientation, and interlayeradhesion that is needed for the routine manufacture of many of thesefilms. Accordingly, there exists a need in the art for an improvedprocess for making coextruded polymeric multilayer optical films withgreater versatility and enhanced control over several steps in themanufacturing process.

SUMMARY OF THE INVENTION

The present invention relates to methods and apparatuses for makingmultilayered optical films.

In brief summary, a useful feedblock useful for making a multilayeroptical film of the invention comprises: (a) a gradient plate comprisingat least first and second flow channels, wherein at least one of theflow channel has a cross-sectional area that changes from a firstposition to a second position along the flow channel; (b) a feeder tubeplate having a first plurality of conduits in fluid communication withthe first flow channel and a second plurality of conduits in fluidcommunication with the second flow channel, each conduit feeding its ownrespective slot die, each conduit having a first end and a second end,the first end of the conduits being in fluid communication with the flowchannels, and the second end of the conduits being in fluidcommunication with the slot die; and (c) an axial rod heater locatedproximal to said conduits.

In brief summary, a method for making a multilayered optical filmcomprises the steps of: (a) providing at least a first and a secondstream of resin; (b) dividing the first and the second streams into aplurality of layers using a feedblock comprising: (i) a gradient platecomprising first and second flow channels, where the first channel has across-sectional area that changes from a first position to a secondposition along the flow channel; (ii) a feeder tube plate having a firstplurality of conduits in fluid communication with the first flow channeland a second plurality of conduits in fluid communication with thesecond flow channel, each conduit feeding its own respective slot die,each conduit having a first end and a second end, the first end of theconduits being in fluid communication with the flow channels, and thesecond end of the conduits being in fluid communication with the slotdie; and (c) an axial rod heater located proximal to said conduits (c)passing the composite stream through an extrusion die to form amultilayer web in which each layer is generally parallel to the majorsurface of adjacent layers; and (d) casting the multilayer web onto acasting roll to form a cast multilayer film.

In brief summary, a method of making a textured multilayer optical filmcomprises the steps of: (a) providing at least a first and a secondstream of resin; (b) dividing the first and the second streams into aplurality of layers such that the layers of the first stream areinterleaved with the layers of the second stream to yield a compositestream; (c) passing the composite stream through an extrusion die toform a multilayer web in which each layer is generally parallel to themajor surface of adjacent layers; (d) casting the multilayer web onto acasting roll; and (e) contacting the multilayer web by a micro-embossingroll to form a cast multilayer film.

In yet another method of making a multilayer optical film, the methodcomprises the steps of: (a) providing at least a first and a secondstream of resin, wherein the first stream of resin is a copolymer ofpolyethylene naphthalate (coPEN) and the second stream of resin ispolymethyl methacrylate (PMMA), (b) dividing the first and the secondstreams into a plurality of layers such that the layers of the firststream are interleaved with the layers of the second stream to yield acomposite stream; (c) coextruding the composite stream through a die toform a multilayer web wherein each layer is generally parallel to themajor surface of adjacent layers, wherein the coPEN and PMMA resins arecoextruded at a melt temperature of about 260° C., and wherein thebirefringence of the coPEN resin is reduced by about 0.02 units or lesscompared to the birefringence of a homopolymer PEN resin for a givendraw ratio; and (d) casting the multilayer web onto a casting roll toform a cast multilayer film.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be more completely understood in consideration of thefollowing detailed description of various embodiments of the inventionin connection with the accompanying drawings, in which:

FIG. 1 is a schematic drawing illustrating the general process usefulfor the coextrusion of multilayered optical films made in accordancewith the present invention;

FIG. 2 is a schematic diagram a portion of the apparatuses useful in theprocess of making a multilayered optical film of the present invention;and

FIG. 3 is a schematic diagram of a feedblock useful in the process ofmaking a multilayered optical film of the present invention.

FIG. 4 is a perspective view of a feedblock similar to that of FIG. 3.

These figures are idealized, are not to scale, and are intended to bemerely illustrative and non-limiting.

DETAILED DESCRIPTION OF THE INVENTION

Various process considerations are important in making high qualitypolymeric multilayer optical films and other optical devices inaccordance with the present invention. Such optical films include, butare not limited to, interference polarizers, mirrors, colored films, andcombinations thereof. The films are optically effective over diverseportions of the ultraviolet, visible, and infrared spectra. Ofparticular interest are coextruded polymeric multilayer optical filmshaving one or more layers that are birefringent in nature. The processconditions used to make each depends in part on (1) the particular resinsystem used and (2) the desired optical properties of the final film.

A preferred method of making the multilayer film of the presentinvention is illustrated schematically in FIG. 1. Materials 100 and 102,selected to have suitably different optical properties, are heated abovetheir melting and/or glass transition temperatures and fed into amultilayer feedblock 104. Typically, melting and initial feeding isaccomplished using an extruder for each material. For example, material100 can be fed into an extruder 101 while material 102 can be fed intoan extruder 103. Exiting from the feedblock 104 is a multilayer flowstream 105. A layer multiplier 106 splits the multilayer flow stream,and then redirects and “stacks” one stream atop the second to multiplythe number of layers extruded. An asymmetric multiplier, when used withextrusion equipment that introduces layer thickness deviationsthroughout the stack, may broaden the distribution of layer thicknessesso as to enable the multilayer film to have layer pairs corresponding toa desired portion of the visible spectrum of light, and provide adesired layer thickness gradient. If desired, skin layers 111 may beintroduced into the multilayer optical film by feeding resin 108 (forskin layers) to a skin layer feedblock 110.

The multilayer feedblock feeds a film extrusion die 112. Feedblocksuseful in the manufacture of the present invention are described in, forexample, U.S. Pat. No. 3,773,882 (Schrenk) and U.S. Pat. No. 3,884,606(Schrenk), the contents of which are incorporated by reference herein.As an example, the extrusion temperature may be approximately 295° C.,and the feed rate approximately 10-150 kg/hour for each material. It isdesirable in most cases to have skin layers 111 flowing on the upper andlower surfaces of the film as it goes through the feedblock and die.These layers serve to dissipate the large stress gradient found near thewall, leading to smoother extrusion of the optical layers. Typicalextrusion rates for each skin layer would be 2-50 kg/hr (1-40% of thetotal throughput). The skin material can be the same material as one ofthe optical layers or be a different material. An extrudate leaving thedie is typically in a melt form.

The extrudate is cooled on a casting wheel 116, which rotates pastpinning wire 114. The pinning wire pins the extrudate to the castingwheel. To achieve a clear film over a broad range of angles, one canmake the film thicker by running the casting wheel at a slow speed,which moves the reflecting band towards longer wavelengths. The film isoriented by stretching at ratios determined by the desired optical andmechanical properties. Longitudinal stretching can be done by pull rolls118. Transverse stretching can be done in a tenter oven 120. If desired,the film can be bi-axially oriented simultaneously. Stretch ratios ofapproximately 3-4 to 1 are preferred, although ratios as small as 2 to 1and as large as 6 to 1 may also be appropriate for a given film. Stretchtemperatures will depend on the type of birefringent polymer used, but2° to 33° C. (5° to 60° F.) above its glass transition temperature wouldgenerally be an appropriate range. The film is typically heat set in thelast two zones 122 of the tenter oven to impart the maximumcrystallinity in the film and reduce its shrinkage. Employing a heat settemperature as high as possible without causing film breakage in thetenter reduces the shrinkage during a heated embossing step. A reductionin the width of the tenter rails by about 1-4% also serves to reducefilm shrinkage. If the film is not heat set, heat shrink properties aremaximized, which may be desirable in some security packagingapplications. The film can be collected on windup roll 124.

In some applications, it may be desirable to use more than two differentpolymers in the optical layers of the multilayer film. In such a case,additional resin streams can be fed using similar means to resin streams100 and 102. A feedblock appropriate for distributing more than twolayer types analogous to the feedblock 104 could be used.

FIG. 2 is a schematic representation of a portion of a typical set-upuseful for the practice of the present invention. Feedblock 200 has foursections: a gradient plate 202, a feeder tube plate 204, an optionalslot plate 206, and optional compression section 208. The slot platehouses a plurality of individual slots, which is a part of the slot die(not shown). Alternatively, the slots can be a part of the feeder tubeplate. The compression section is typically located in the feedblock,although it does not need to be. Adjacent to the feedblock is a unit 210useful for the introduction of protective boundary layers. Althoughmultipliers 212 and 214 are shown, it is within the scope of thisinvention to use no multipliers or at least one multiplier to increasethe number of layers in the multilayer optical film. A unit 216 isuseful for the introduction of skin layers, if desired. A film castingdie 218 begins the formation of the multilayer film. Extrudate exitingthe casting die is allowed to contact a casting wheel 220. The castingwheel is typically cooled to quench the extrudate and form a film.Additional processing, such as drawing, orienting, and heat-setting theinventive multilayer film can also be done.

The above description is intended to provide an overview of the methodand apparatus encompassed within the present invention. It should beunderstood, however, that the intention is not to limit the invention tothe particular embodiments described, but to cover all modifications,equivalents, and alternatives falling within the spirit and scope of theinvention as defined by the appended claims.

Material Selection

A variety of polymer materials suitable for use in the present inventionhave been taught for use in making coextruded multilayer optical films.For example, the polymer materials listed and described in U.S. Pat.Nos. 4,937,134, 5,103,337, 5,1225,448,404, 5,540,978, and 5,568,316 toSchrenk et al., and in U.S. Pat. Nos. 5,122,905, 5,122,906, and5,126,880 to Wheatley and Schrenk are useful for making multilayeroptical films according to the present invention. Of special interestare birefringent polymers such as those described in U.S. Pat. Nos.5,486,949 and 5,612,820 to Schrenk et al; in U.S. patent applicationSer. No. 08/402,041 to Jonza et al.; and in U.S. patent application Ser.No. 09/006,601 entitled “Modified Copolyesters and Improved MultilayerReflective Films” filed Jan. 13, 1998, are of which are incorporated byreference. Regarding the preferred materials from which the films are tobe made, there are several conditions which should be met to make themultilayer optical films of this invention. First, these films shouldconsist of at least two distinguishable polymers. The number of polymersis not limited, and three or more polymers may be advantageously used inparticular films. Second, at least one of the two required polymers,commonly referred to as the “first polymer,” preferably has a stressoptical coefficient having a large absolute value. In other words, thefirst polymer preferably develops a large birefringence when stretched.Depending on the application of the multilayer film, the birefringencemay be developed between two orthogonal directions in the plane of thefilm, between one or more in-plane directions and the directionperpendicular to the film plane, or a combination of these. In thespecial case that the isotropic indices are widely separated, thepreference for large birefringence in the first polymer may be relaxed,although birefringence is still usually desirable. Such special casesmay arise in the selection of polymers for mirror films and forpolarizer films formed using a biaxial process, which draws the film intwo orthogonal in-plane directions. Third, the first polymer should becapable of maintaining birefringence after stretching, so that thedesired optical properties are imparted to the finished film. Fourth,the other required polymer, commonly referred to as the “secondpolymer,” should be chosen so that in the finished film, its refractiveindex, in at least one direction, differs significantly from the indexof refraction of the first polymer in the same direction. Becausepolymeric materials are typically dispersive, that is, the refractiveindices vary with wavelength, these conditions must be considered interms of a particular spectral bandwidth of interest.

Other aspects of polymer selection depend on specific applications. Forpolarizing films, it is advantageous for the difference in the index ofrefraction of the first and second polymers in one film-plane directionto differ significantly in the finished film, while the difference inthe orthogonal film-plane index is minimized. If the first polymer has alarge refractive index when isotropic, and is positively birefringent(that is, its refractive index increases in the direction ofstretching), the second polymer will typically be chosen to have amatching refractive index, after processing, in the planar directionorthogonal to the stretching direction, and a refractive index in thedirection of stretching which is as low as possible. Conversely, if thefirst polymer has a small refractive index when isotropic, and isnegatively birefringent, the second polymer will typically be chosen tohave a matching refractive index, after processing, in the planardirection orthogonal to the stretching direction, and a refractive indexin the direction of stretching which is as high as possible.

Alternatively, it is possible to select a first polymer which ispositively birefringent and has an intermediate or low refractive indexwhen isotropic, or one which is negatively birefringent and has anintermediate or high refractive index when isotropic. In these cases,the second polymer may typically be chosen so that, after processing,its refractive index will match that of the first polymer in either thestretching direction or the planar direction orthogonal to stretching.Further, the second polymer will typically be chosen such that thedifference in index of refraction in the remaining planar direction ismaximized, regardless of whether this is best accomplished by a very lowor very high index of refraction in that direction.

There are several means to achieve the combination of planar indexmatching in one direction and mismatching in the orthogonal direction.For example, one can select a first polymer which develops significantbirefringence when stretched, select a second polymer which developslittle or no birefringence when stretched, and to stretch the resultingfilm in only one planar direction. In another method, the second polymercan be selected from among those which develop birefringence in thesense opposite to that of the first polymer (negative-positive orpositive-negative). Another method involves selecting both first andsecond polymers which are capable of developing birefringence whenstretched, but to stretch the multilayer film in two orthogonal planardirections. This latter method involves selecting process conditions(such as temperatures, stretch rates, post-stretch relaxation, and thelike) that result in the development of unequal levels of orientation inthe two stretching directions for the first and second polymers, suchthat one in-plane index is approximately matched to that of the firstpolymer, and the orthogonal in-plane index is significantly mismatchedto that of the first polymer. For example, conditions may be chosen suchthat the first polymer has a biaxially oriented character in thefinished film, while the second polymer has a predominantly uniaxiallyoriented character in the finished film.

The foregoing discussion for polarizing film is meant to be exemplary.It will be understood that combinations of these and other techniquesmay be used to achieve the index mismatch in one in-plane direction andrelative index matching in the orthogonal planar direction.

Different considerations apply to a reflective, or mirror, film.Provided that the film is not meant to have some polarizing propertiesas well, refractive index criteria apply equally to any direction in thefilm plane. Thus, typical for the indices for any given layer inorthogonal in-plane directions to be nearly equal. It is advantageous,however, for the film-plane indices of the first polymer to differ asgreatly as possible from the film-plane indices of the second polymer.For this reason, if the first polymer has a high index of refractionwhen isotropic, it is advantageous that it also be positivelybirefringent. Likewise, if the first polymer has a low index ofrefraction when isotropic, it is advantageous that it also be negativelybirefringent. The second polymer advantageously develops little or nobirefringence when stretched, or develops birefringence of the oppositesense (positive-negative or negative-positive), such that its film-planerefractive indices differ as much as possible from those of the firstpolymer in the finished film. These criteria may be combinedappropriately with those listed above for polarizing films if a mirrorfilm is meant to have some degree of polarizing properties as well.

Colored films can be regarded as special cases of mirror and polarizingfilms. Thus, the same criteria outlined above apply. The perceived coloris a result of reflection or polarization over one or more specificbandwidths of the spectrum. The bandwidths over which a multilayer filmof the current invention is effective will be determined primarily bythe distribution of layer thicknesses used in the optical stack(s), butconsideration must also be given to the wavelength dependence, ordispersion, of the refractive indices of the first and second polymers.It will be understood that the same rules apply to the infrared andultraviolet wavelengths as to the visible colors.

Absorbance is another consideration. For most applications, it isadvantageous for neither the first nor the second polymer to have anyabsorbance bands within the bandwidth of interest for the film inquestion. Thus, all incident light within the bandwidth is eitherreflected or transmitted. However, for some applications, it may beuseful for one or both of the first and second polymer to absorbspecific wavelengths, either totally or in part.

Although many polymers may be chosen as the first polymer, certain ofthe polyesters have the capability for particularly large birefringence.Among these, polyethylene 2,6-naphthalate (PEN) is frequently chosen asa first polymer for films of the present invention. It has a very largepositive stress optical coefficient, retains birefringence effectivelyafter stretching, and has little or no absorbance within the visiblerange. It also has a large index of refraction in the isotropic state.Its refractive index for polarized incident light of 550 nm wavelengthincreases when the plane of polarization is parallel to the stretchdirection from about 1.64 to as high as about 1.9. Its birefringence canbe increased by increasing its molecular orientation which, in turn, maybe increased by stretching to greater stretch ratios with otherstretching conditions held fixed.

Other semicrystalline naphthalene dicarboxylic polyesters are alsosuitable as first polymers. Polybutylene 2,6-Naphthalate (PBN) is anexample. These polymers may be homopolymers or copolymers, provided thatthe use of comonomers does not substantially impair the stress opticalcoefficient or retention of birefringence after stretching. The term“PEN” herein will be understood to include copolymers of PEN meetingthese restrictions. In practice, these restrictions imposes an upperlimit on the comonomer content, the exact value of which will vary withthe choice of comonomer(s) employed. Some compromise in these propertiesmay be accepted, however, if comonomer incorporation results inimprovement of other properties. Such properties include but are notlimited to improved interlayer adhesion, lower melting point (resultingin lower extrusion temperature), better rheological matching to otherpolymers in the film, and advantageous shifts in the process window forstretching due to change in the glass transition temperature.

Suitable comonomers for use in PEN, PBN or the like may be of the diolor dicarboxylic acid or ester type. Dicarboxylic acid comonomers includebut are not limited to terephthalic acid, isophthalic acid, phthalicacid, all isomeric naphthalenedicarboxylic acids (2,6-, 1,2-, 1,3-,1,4-, 1,5-, 1,6-, 1,7-, 1,8-, 2,3-, 2,4-, 2,5-, 2,7-, and 2,8-),bibenzoic acids such as 4,4′-biphenyl dicarboxylic acid and its isomers,trans-4,4′-stilbene dicarboxylic acid and its isomers, 4,4′-diphenylether dicarboxylic acid and its isomers, 4,4′-diphenylsulfonedicarboxylic acid and its isomers, 4,4′-benzophenone dicarboxylic acidand its isomers, halogenated aromatic dicarboxylic acids such as2-chloroterephthalic acid and 2,5-dichloroterephthalic acid, othersubstituted aromatic dicarboxylic acids such as tertiary butylisophthalic acid and sodium sulfonated isophthalic acid, cycloalkanedicarboxylic acids such as 1,4-cyclohexanedicarboxylic acid and itsisomers and 2,6-decahydronaphthalene dicarboxylic acid and its isomers,bi- or multi-cyclic dicarboxylic acids (such as the various isomericnorbornene and norbornene dicarboxylic acids, adamantane dicarboxylicacids, and bicyclo-octane dicarboxylic acids), alkane dicarboxylic acids(such as sebacic acid, adipic acid, oxalic acid, malonic acid, succinicacid, glutaric acid, azelaic acid, and dodecane dicarboxylic acid.), andany of the isomeric dicarboxylic acids of the fused-ring aromatichydrocarbons (such as indene, anthracene, pheneanthrene, benzonaphthene,fluorene and the like). Alternatively, alkyl esters of these monomers,such as dimethyl terephthalate, may be used.

Suitable diol comonomers include but are not limited to linear orbranched alkane diols or glycols (such as ethylene glycol, propanediolssuch as trimethylene glycol, butanediols such as tetramethylene glycol,pentanediols such as neopentyl glycol, hexanediols,2,2,4-trimethyl-1,3-pentanediol and higher diols), ether glycols (suchas diethylene glycol, triethylene glycol, and polyethylene glycol),chain-ester diols such as3-hydroxy-2,2-dimethylpropyl-3-hydroxy-2,2-dimethyl propanoate,cycloalkane glycols such as 1,4-cyclohexanedimethanol and its isomersand 1,4-cyclohexanediol and its isomers, bi- or multicyclic diols (suchas the various isomeric tricyclodecane dimethanols, norbornanedimethanols, norbornene dimethanols, and bicyclo-octane dimethanols),aromatic glycols (such as 1,4-benzenedimethanol and its isomers,1,4-benzenediol and its isomers, bisphenols such as bisphenol A,2,2′-dihydroxy biphenyl and its isomers, 4,4′-dihydroxymethyl biphenyland its isomers, and 1,3-bis(2-hydroxyethoxy)benzene and its isomers),and lower alkyl ethers or diethers of these diols, such as dimethyl ordiethyl diols.

Tri- or polyfunctional comonomers, which can serve to impart a branchedstructure to the polyester molecules, can also be used. They may be ofeither the carboxylic acid, ester, hydroxy or ether types. Examplesinclude, but are not limited to, trimellitic acid and its esters,trimethylol propane, and pentaerythritol.

Also suitable as comonomers are monomers of mixed functionality,including hydroxycarboxylic acids such as parahydroxybenzoic acid and6-hydroxy-2-naphthalenecarboxylic acid, and their isomers, and tri- orpolyfunctional comonomers of mixed functionality such as5-hydroxyisophthalic acid and the like.

Polyethylene terephthalate (PET) is another material that exhibits asignificant positive stress optical coefficient, retains birefringenceeffectively after stretching, and has little or no absorbance within thevisible range. Thus, it and its high PET-content copolymers employingcomonomers listed above may also be used as first polymers in someapplications of the current invention.

When a naphthalene dicarboxylic polyester such as PEN or PBN is chosenas first polymer, there are several approaches which may be taken to theselection of a second polymer. One preferred approach for someapplications is to select a naphthalene dicarboxylic copolyester (coPEN)formulated so as to develop significantly less or no birefringence whenstretched. This can be accomplished by choosing comonomers and theirconcentrations in the copolymer such that crystallizability of the coPENis eliminated or greatly reduced. One typical formulation employs as thedicarboxylic acid or ester components dimethyl naphthalate at from about20 mole percent to about 80 mole percent and dimethyl terephthalate ordimethyl isophthalate at from about 20 mole percent to about 80 molepercent, and employs ethylene glycol as diol component. Of course, thecorresponding dicarboxylic acids maybe used instead of the esters. Thenumber of comonomers which can be employed in the formulation of a coPENsecond polymer is not limited. Suitable comonomers for a coPEN secondpolymer include but are not limited to all of the comonomers listedabove as suitable PEN comonomers, including the acid, ester, hydroxy,ether, tri- or polyfunctional, and mixed functionality types.

Often it is useful to predict the isotropic refractive index of a coPENsecond polymer. A volume average of the refractive indices of themonomers to be employed has been found to be a suitable guide. Similartechniques well-known in the art can be used to estimate glasstransition temperatures for coPEN second polymers from the glasstransitions of the homopolymers of the monomers to be employed.

In addition, polycarbonates having a glass transition temperaturecompatible with that of PEN and having a refractive index similar to theisotropic refractive index of PEN are also useful as second polymers.Polyesters, copolyesters, polycarbonates, and copolycarbonates may alsobe fed together to an extruder and transesterified into new suitablecopolymeric second polymers.

It is not required that the second polymer be a copolyester orcopolycarbonate. Vinyl polymers and copolymers made from monomers suchas vinyl naphthalenes, styrenes, ethylene, maleic anhydride, acrylates,acetates, and methacrylates may be employed. Condensation polymers otherthan polyesters and polycarbonates may also be used. Examples include:polysulfones, polyamides, polyurethanes, polyamic acids, and polyimides.Naphthalene groups and halogens such as chlorine, bromine and iodine areuseful for increasing the refractive index of the second polymer to adesired level. Acrylate groups and fluorine are particularly useful indecreasing refractive index when this is desired.

It will be understood from the foregoing discussion that the choice of asecond polymer is dependent not only on the intended application of themultilayer optical film in question, but also on the choice made for thefirst polymer, and the processing conditions employed in stretching.Suitable second polymer materials include but are not limited topolyethylene naphthalate (PEN) and isomers thereof (such as 2,6-, 1,4-,1,5-, 2,7-, and 2,3-PEN), polyalkylene terephthalates (such aspolyethylene terephthalate, polybutylene terephthalate, andpoly-1,4-cyclohexanedimethylene terephthalate), other polyesters,polycarbonates, polyarylates, polyamides (such as nylon 6, nylon 11,nylon 12, nylon 4/6, nylon 6/6, nylon 6/9, nylon 6/10, nylon 6/12, andnylon 6/T), polyimides (including thermoplastic polyimides andpolyacrylic imides), polyamide-imides, polyether-amides,polyetherimides, polyaryl ethers (such as polyphenylene ether and thering-substituted polyphenylene oxides), polyarylether ketones such aspolyetheretherketone (“PEEK”), aliphatic polyketones (such as copolymersand terpolymers of ethylene and/or propylene with carbon dioxide),polyphenylene sulfide, polysulfones (including polyethersulfones andpolyaryl sulfones), atactic polystyrene, syndiotactic polystyrene(“sPS”) and its derivatives (such as syndiotactic poly-alpha-methylstyrene and syndiotactic polydichlorostyrene), blends of any of thesepolystyrenes (with each other or with other polymers, such aspolyphenylene oxides), copolymers of any of these polystyrenes (such asstyrene-butadiene copolymers, styrene-acrylonitrile copolymers, andacrylonitrile-butadiene-styrene terpolymers), polyacrylates (such aspolymethyl acrylate, polyethyl acrylate, and polybutyl acrylate),polymethacrylates (such as polymethyl methacrylate, polyethylmethacrylate, polypropyl methacrylate, and polyisobutyl methacrylate),cellulose derivatives (such as ethyl cellulose, cellulose acetate,cellulose propionate, cellulose acetate butyrate, and cellulosenitrate), polyalkylene polymers (such as polyethylene, polypropylene,polybutylene, polyisobutylene, and poly(4-methyl)pentene), fluorinatedpolymers and copolymers (such as polytetrafluoroethylene,polytrifluoroethylene, polyvinylidene fluoride, polyvinyl fluoride,fluorinated ethylene-propylene copolymers, perfluoroalkoxy resins,polychlorotrifluoro ethylene, polyethylene-co-tri fluoro ethylene,polyethylene-co-chlorotrifluoroethylene), chlorinated polymers (such aspolyvinylidene chloride and polyvinyl chloride), polyacrylonitrile,polyvinylacetate, polyethers (such as polyoxymethylene and polyethyleneoxide), ionomeric resins, elastomers (such as polybutadiene,polyisoprene, and neoprene), silicone resins, epoxy resins, andpolyurethanes.

Also suitable are copolymers, such as the copolymers of PEN discussedabove as well as any other non-naphthalene group-containing copolyesterswhich may be formulated from the above lists of suitable polyestercomonomers for PEN. In some applications, especially when PET serves asthe first polymer, copolyesters based on PET and comonomers from saidlists above (coPETs) are especially suitable. In addition, either firstor second polymers may consist of miscible or immiscible blends of twoor more of the above-described polymers or copolymers (such as blends ofsPS and atactic polystyrene, or of PEN and sPS). The coPENs and coPETsdescribed may be synthesized directly, or may be formulated as a blendof pellets where at least one component is a polymer based onnaphthalene dicarboxylic acid or terephthalic acid and other componentsare polycarbonates or other polyesters, such as a PET, a PEN, a coPET,or a co-PEN.

Another preferred family of materials for the second polymer for someapplications are the syndiotactic vinyl aromatic polymers, such assyndiotactic polystyrene. Syndiotactic vinyl aromatic polymers useful inthe current invention include poly(styrene), poly(alkyl styrene)s, poly(aryl styrene)s, poly(styrene halide)s, poly(alkoxy styrene)s,poly(vinyl ester benzoate), poly(vinyl naphthalene), poly(vinylstyrene),and poly(acenaphthalene), as well as the hydrogenated polymers andmixtures or copolymers containing these structural units. Examples ofpoly(alkyl styrene)s include the isomers of the following: poly(methylstyrene), poly(ethyl styrene), poly(propyl styrene), and poly(butylstyrene). Examples of poly(aryl styrene)s include the isomers ofpoly(phenyl styrene). As for the poly(styrene halide)s, examples includethe isomers of the following: poly(chlorostyrene), poly(bromostyrene),and poly(fluorostyrene). Examples of poly(alkoxy styrene)s include theisomers of the following: poly(methoxy styrene) and poly(ethoxystyrene). Among these examples, particularly preferable styrene grouppolymers, are: polystyrene, poly(p-methyl styrene), poly(m-methylstyrene), poly(p-tertiary butyl styrene), poly(p-chlorostyrene),poly(m-chloro styrene), poly(p-fluoro styrene), and copolymers ofstyrene and p-methyl styrene.

Furthermore, comonomers may be used to make syndiotactic vinyl aromaticgroup copolymers. In addition to the monomers for the homopolymerslisted above in defining the syndiotactic vinyl aromatic polymers group,suitable comonomers include olefin monomers (such as ethylene,propylene, butenes, pentenes, hexenes, octenes or decenes), dienemonomers (such as butadiene and isoprene), and polar vinyl monomers(such as cyclic diene monomers, methyl methacrylate, maleic acidanhydride, or acrylonitrile).

The syndiotactic vinyl aromatic copolymers of the present invention maybe block copolymers, random copolymers, or alternating copolymers.

The syndiotactic vinyl aromatic polymers and copolymers referred to inthis invention generally have syndiotacticity of higher than 75% ormore, as determined by carbon-13 nuclear magnetic resonance. Preferably,the degree of syndiotacticity is higher than 85% racemic diad, or higherthan 30%, or more preferably, higher than 50%, racemic pentad.

In addition, although there are no particular restrictions regarding themolecular weight of these syndiotactic vinyl aromatic polymers andcopolymers, preferably, the weight average molecular weight is greaterthan 10,000 and less than 1,000,000, and more preferably, greater than50,000 and less than 800,000.

The syndiotactic vinyl aromatic polymers and copolymers may also be usedin the form of polymer blends with, for instance, vinyl aromatic grouppolymers with atactic structures, vinyl aromatic group polymers withisotactic structures, and any other polymers that are miscible with thevinyl aromatic polymers. For example, polyphenylene ethers show goodmiscibility with many of the previous described vinyl aromatic grouppolymers.

When a polarizing film is made using a process with predominantlyuniaxial stretching, particularly preferred combinations of polymers foroptical layers include PEN/coPEN, PET/coPET, PEN/sPS, PET/sPS,PEN/Eastar,™ and PET/Eastar,™ where “coPEN” refers to a copolymer orblend based upon naphthalene dicarboxylic acid (as described above) andEastar™ is a polyester or copolyester (believed to comprisecyclohexanedimethylene diol units and terephthalate units) commerciallyavailable from Eastman Chemical Co. When a polarizing film is to be madeby manipulating the process conditions of a biaxial stretching process,particularly preferred combinations of polymers for optical layersinclude PEN/coPEN, PEN/PET, PEN/PBT, PEN/PETG and PEN/PETcoPBT, where“PBT” refers to polybutylene terephthalate, “PETG” refers to a copolymerof PET employing a second glycol (usually cyclohexanedimethanol), and“PETcoPBT” refers to a copolyester of terephthalic acid or an esterthereof with a mixture of ethylene glycol and 1,4-butanediol.

Particularly preferred combinations of polymers for optical layers inthe case of mirrors or colored films include PEN/PMMA, PET/PMMA,PEN/Ecdel,™ PET/Ecdel,™ PEN/sPS, PET/sPS, PEN/coPET, PEN/PETG, andPEN/THV,™ where “PMMA” refers to polymethyl methacrylate, Ecdel™ is athermoplastic polyester or copolyester (believed to comprisecyclohexanedicarboxylate units, polytetramethylene ether glycol units,and cyclohexanedimethanol units) commercially available from EastmanChemical Co., “coPET” refers to a copolymer or blend based uponterephthalic acid (as described above), “PETG” refers to a copolymer ofPET employing a second glycol (usually cyclohexanedimethanol), and THV™is a fluoropolymer commercially available from 3M Co.

For mirror films, a match of the refractive indices of the first polymerand second polymer in the direction normal to the film plane issometimes preferred, because it provides for constant reflectance withrespect to the angle of incident light (that is, there is no Brewster'sangle). For example, at a specific wavelength, the in-plane refractiveindices might be 1.76 for biaxially oriented PEN, while the filmplane-normal refractive index might fall to 1.49. When PMMA is used asthe second polymer in the multilayer construction, its refractive indexat the same wavelength, in all three directions, might be 1.495. Anotherexample is the PET/Ecdel™ system, in which the analogous indices mightbe 1.66 and 1.51 for PET, while the isotropic index of Ecdel™ might be1.52. The crucial property is that the normal-to-plane index for onematerial must be closer to the in-plane indices of the other materialthan to its own in-plane indices.

In other embodiments, a deliberate mismatching of the normal-to-planerefractive index is desirable. Some examples include those involvingthree or more polymeric layers in the optical stack in which adeliberate mismatch in the normal-to-plane index is desirable oppositein sign to the index mismatch in one of the in-plane directions. It issometimes preferred for the multilayer optical films of the currentinvention to consist of more than two distinguishable polymers. A thirdor subsequent polymer might be fruitfully employed as anadhesion-promoting layer between the first polymer and the secondpolymer within an optical stack, as an additional component in a stackfor optical purposes, as a protective boundary layer between opticalstacks, as a skin layer, as a functional coating, or for any otherpurpose. As such, the composition of a third or subsequent polymer, ifany, is not limited. Some preferred multicomponent constructions aredescribed in U.S. patent application Ser. No. 09/006,118 filed Jan. 13,1998 entitled “Multicomponent Optical Body,” the contents of which areherein incorporated by reference.

Process Considerations

The process used for making the coextruded polymeric multilayer opticalfilms of the present invention will vary depending on the resinmaterials selected and the optical properties desired in the finishedfilm product.

Moisture sensitive resins should be dried before or during extrusion toprevent degradation. The drying can be done by any means known in theart. One well-known means employs ovens or more sophisticated heatedvacuum and/or desiccant hopper-dryers to dry resin prior to its beingfed to an extruder. Another means employs a vacuum-vented twin-screwextruder to remove moisture from the resin while it is being extruded.Drying time and temperature should be limited to prevent thermaldegradation or sticking during hopper-dryer or oven drying. In addition,resins coextruded with moisture sensitive resins should be dried toprevent damage to the moisture sensitive coextruded resin from moisturecarried by the other resin.

Extrusion conditions are chosen to adequately feed, melt, mix and pumpthe polymer resin feed streams in a continuous and stable manner. Finalmelt stream temperatures are chosen within a range which avoidsfreezing, crystallization or unduly high pressure drops at the low endof the temperature range and which avoids degradation at the high end ofthe temperature range. For example, polyethylene naphthalate (PEN) isdried for 8 hours at 135° C. and then vacuum fed to an extruder with afinal zone temperature, or melt temperature, ranging preferably between270° C. and 300° C. and more preferably between 275° C. and 290° C.

It is often preferable for all polymers entering the multilayerfeedblock to be at the same or very similar melt temperatures. This mayrequire process compromise if two polymers, whose ideal melt processingtemperatures do not match, are to be coextruded. For example, polymethylmethacrylate (PMMA) is typically extruded at a temperature below about250° C. Applicants have found, however, that PMMA can be coextruded withPEN using PMMA melt temperatures as high as 275° C., provide that designconsiderations are made in the PMMA melt train to minimize the potentialfor stagnation points in the flow, and to hold to a minimum the overallresidence time in the melt of the PMMA. Another technique found to beuseful in this regard is to start up the PMMA melt train at the moreconventional processing temperatures, and then to raise the melt traintemperatures to the higher, PEN-compatible temperatures only whenwell-developed flow through the entire process has been attained.

Conversely, the PEN processing temperature may be reduced so as to matchit to the typical melt processing temperatures for PMMA. Thus, it hasalso been found that the melting point, and hence, the processingtemperature, of PEN may be reduced by the addition of comonomers intothe PEN polymer with only a very slight accompanying reduction of theability of the PEN to develop birefringence upon drawing. For example, aPEN copolymer made using DiMethyl Isophthalate (DMI) in place of 3 mol %of the 2,6-DiMethyl Naphthalate (DMN) monomer has been found to have areduction in birefringence of only 0.02 units, and a reduction of glasstransition temperature of only about 4 or 5° C., while the meltprocessing temperature is reduced by 15° C. Small amounts of DiMethylTerephthalate (DMT) or other diacid or diol comonomers may also beusefull in this regard. Esters or diesters of the diacid comonomers mayalso be used. The advantages of adding comonomers into the PEN polymerare more fully described in U.S. patent applications Ser. No. 09/006,601entitled “Modified Copolyesters and Improved Multilayer Reflective Film”and Ser. No. 09/006,468 entitled “Optical Device with a DichroicPolarizer and Multilayer Optical Film,” both filed on filed Jan. 13,1998, the contents of which are incorporated herein by reference.

It will be evident to one skilled in the art that combinations of PENprocess temperature reduction through copolymerization and PMMA melttemperature elevation via process design could be usefully employed, ascould the combination of one, the other, or both techniques with stillother techniques. Likewise, similar techniques could be employed forequal-temperature coextrusion of PEN with polymers other than PMMA, PMMAwith polymers other than PEN, or combinations including neither of thetwo exemplary polymers.

Following extrusion, the melt streams are then filtered to removeundesirable particles and gels. Primary and secondary filters known inthe art of polyester film manufacture may be used, with mesh sizes inthe 1-30 micrometer range. While the prior art indicates the importanceof such filtration to film cleanliness and surface properties, itssignificance in the present invention extends to layer uniformity aswell. Each melt stream is then conveyed through a neck tube into a gearpump used to regulate the continuous and uniform rate of polymer flow. Astatic mixing unit may be placed at the end of the neck tube carryingthe melt from the gear pump into the multilayer feedblock, in order toensure uniform melt stream temperature. The entire melt stream is heatedas uniformly as possible to ensure both uniform flow and minimaldegradation during processing.

Multilayer feedblocks are designed to divide two or more polymer meltstreams into many layers each, interleave these layers, and merge themany layers of two or more polymers into a single multilayer stream. Thelayers from any given melt stream are created by sequentially bleedingoff part of the stream from a flow channel into side channel tubes thatfeed layer slots for the individual layers in the feedblock. Manydesigns are possible, including those disclosed in U.S. Pat. Nos.3,737,882; 3,884,606; and 3,687,589 to Schrenk et al. Methods have alsobeen described to introduce a layer thickness gradient by controllinglayer flow as described in U.S. Pat. Nos. 3,195,865; 3,182,965;3,051,452; 3,687,589 and 5,094,788 to Schrenk et al, and in U.S. Pat.No. 5,389,324 to Lewis et al. In typical industrial processes, layerflow is generally controlled by choices made in machining the shape andphysical dimensions of the individual side channel tubes and layerslots.

Applicants have discovered an improved feedblock design that allows forbetter control of the layer thickness distribution and of the layeruniformity. The improved design incorporates modular features so thatonly a few sections of the feedblock need to be machined for each uniquefilm construction, as further described below. The economic advantage ofthe modular design is reduction in time, labor, and equipment needed tochange from one film construction to another.

FIG. 3 shows a schematic cross-section feedblock 10, which is enclosedin a housing 12. Within the housing 12 reside an optional manifold plate20 and a gradient plate 30, which in combination, define at least twosupplemental channels, a first channel 22 and a second channel 24. Asshown, a portion of the bottom surface of manifold plate 20 togetherwith a portion of the top surface of gradient plate 30 define thesupplemental channels 22 and 24. The supplemental channels are anoptional feature of the feedblock, and they help convey resin from oneposition in the feedblock to another position. In addition, plate-typeheaters (not shown) can be attached to the external surfaces of thehousing 12.

Residing in gradient plate 30 are at least two flow channels, a firstflow channel 32 and a second flow channel 34. The flow channels arebounded by a combination of the gradient plate 30 and a feeder tubeplate 40. The first flow channel 32 is in fluid communication with thefirst supplemental channel 22 while second flow channel 34 is in fluidcommunication with second supplemental channel 24. When supplementalchannels are used in combination with flow channels, transfer conduits(not shown) serve as the communication means to connect the two types ofchannels together. Although only a pair of supplemental channels and apair of flow channels are shown, it is within the scope of thisinvention to use more than two channels of each type.

In the gradient plate 30, each flow channel is machined so that itscross-section has a central axis of symmetry, such as, e.g., a circle,square, or equilateral triangle. For ease of machining purposes, thesquare cross-section flow channel is preferably used. Along each flowchannel, the cross-sectional area can remain constant or can change. Thechange may be an increase or decrease in area, and a decreasingcross-section is typically referred to as a “taper.” A change incross-sectional area of the flow channels can be designed to provide anappropriate pressure gradient, which affects the layer thicknessdistribution of a multilayer optical film. Thus, the gradient plate canbe changed for different types of multilayer film constructions.

When the cross-sectional area of the flow channels is made to remainconstant, a plot of layer thickness vs. layer number is non-linear anddecreasing. For a given polymer flow, there exists at least onecross-sectional tapering profile which will result in a linear,decreasing dependency of layer thickness upon layer number, which issometimes preferred. The taper profile can be found by one reasonablyskilled in the art using reliable rheological data for the polymer inquestion and polymer flow simulation software known in the art, andshould be calculated on a case-by-case basis.

Referring again to FIG. 3, the feedblock 10 further contains a feedertube plate 40 that has a first set of conduits 42 and a second set ofconduits 44, each set in fluid communication with flow channels 32 and34 respectively. As used in this document, the “conduits” are alsoreferred to as “side channel tubes.” Optionally, residing in between thetwo sets of conduits is an axial rod heater 46, used to provide heat tothe resin flowing in the conduits. If desired, temperature can be variedin zones along the length of the axial rod heater. Additional axial rodheaters can be used, for example, one adjacent to conduit 42 and anotheradjacent to conduit 44. Each conduit feeds its own respective slot die56, which has an expansion section and a slot section. The expansionsection typically resides in the feeder tube plate 40. If desired, theslot section can reside in a slot plate 50. As used in this document,the term “slot die” is synonymous with “layer slot.” The first set ofconduits 42 is interleaved with the second set of conduits 44 to formalternating layers.

In use, polymeric resins, in the form of a melt stream, are delivered tothe supplemental channels 22 and 24, if present, from a source, such asan extruder. Typically, a different resin is delivered to eachsupplemental channel. For example, resin A is delivered to channel 22and resin B is delivered to channel 24 as two distinct melt streams. Ifsupplemental channels are not used, resin A and resin B would bedelivered directly to the flow channels 32 and 34. As the melt stream Aand melt stream B travel down the flow channels in the gradient plate30, each melt stream is bled off by the conduits. Because the conduits42 and 44 are interleaved, they begin the formation of alternatinglayers, such as, for example, ABABAB. Each conduit has its own slot dieto begin the formation of an actual layer. The melt stream exiting theslot die contains a plurality of alternating layers. The melt stream isfed into a compression section (not shown) where the layers arecompressed and also uniformly spread out transversely. Special thicklayers known as protective boundary layers (PBLs) may be fed nearest tothe feedblock walls from any of the melt streams used for the opticalmultilayer stack. The PBLs can also be fed by a separate feed streamafter the feedblock. The PBLs function to protect the thinner opticallayers from the effects of wall stress and possible resulting flowinstabilities.

In optical applications, especially for films intended to transmit orreflect a specific color(s), very precise layer thickness uniformity inthe film plane is required. Perfect layer uniformity following atransverse spreading step, occurring in the slot die, is difficult toachieve in practice. The greater the amount of transverse spreadingrequired, the higher the likelihood of non-uniformity in the resultinglayer thickness profile. Thus, it is advantageous from the standpoint oflayer thickness profile uniformity (or for film color uniformity) forthe feedblock's slot die to be relatively wide. However, increasing thewidths of the slot die results in a larger, heavier, and more expensivefeedblock. It will be apparent that an assessment of the optimal slotwidths must be made individually for each feedblock case, taking intoconsideration the optical uniformity requirements of the resulting film.This assessment can be done using reliable Theological data for thepolymer in question and polymer flow simulation software known in theart, along with a model for feedblock fabrication costs.

Control of layer thickness is especially useful in producing filmshaving specific layer thicknesses or thickness gradient profiles thatare modified in a prescribed way throughout the thickness of themultilayer film. For example, several layer thickness designs have beendescribed for infrared films, which minimize higher order harmonics.Such harmonics can cause color in the visible region of the spectrum.Examples of such film include those described in U.S. Pat. No. RE34,605, incorporated herein by reference, which describes a multilayeroptical interference film comprising three diverse substantiallytransparent polymeric materials, A, B, and C and having a repeating unitof ABCB. The layers have an optical thickness of between about 0.09 and0.45 micrometers, and each of the polymeric materials has a differentindex of refraction, n_(i). The film includes polymeric layers ofpolymers A, B, and C. Each of the polymeric materials has its owndifferent refractive index, n_(A), n_(B), n_(C), respectively. Apreferred relationship of the optical thickness ratios of the polymersproduces an optical interference film in which multiple successivehigher order reflections are suppressed. In this embodiment, the opticalthickness ratio of first material A, f_(A), is ⅕, the optical thicknessratio of second material B, f_(B), is ⅙, the optical thickness of thirdmaterial C, f_(C) is ⅓, and n_(B)=√{square root over (n_(A)n_(C))}.

For this embodiment, there will be an intense reflection at the firstorder wavelength, while the reflections at the second, third, and fourthorder wavelengths will be suppressed. To produce a film that reflects abroad bandwidth of wavelengths in the solar infrared range (e.g.,reflection at from about 0.7 to 2.0 micrometers), a layer thicknessgradient may be introduced across the thickness of the film. Forexample, the layer thicknesses may increase monotonically across thethickness of the film. Preferably, in a three component system of thepresent invention, the first polymeric material (A) differs inrefractive index from the second polymeric material (B) by at leastabout 0.03, the second polymeric material (B) differs in refractiveindex from the third polymeric material (C) by at least about 0.03, andthe refractive index of the second polymeric material (B) isintermediate the respective refractive indices of the first (A) andthird (C) polymeric materials. Polymeric materials can be synthesized tohave the desired index of refraction by using a copolymer or miscibleblend of polymers. For example, the second polymeric material may be acopolymer or miscible blend of the first and third polymeric materials.By varying the relative amounts of monomers in the copolymer or polymersin the blend, any of the first, second, or third materials can beadjusted so that there is a refractive index relationship wheren_(B)=√{square root over (n_(A)n_(C))}.

Another suitable film is described in U.S. Pat. No. 5,360,659,incorporated herein by reference. The patent describes a two componentfilm having six layers alternating repeating unit. The film suppressesthe unwanted second, third, and fourth order reflections in the visiblewavelength region of between about 380-770 nm while reflecting light inthe infrared wavelength region of between about 770-2000 nm. Reflectionshigher than fourth order will generally be in the ultraviolet, notvisible, region of the spectrum or will be of such a low intensity as tobe unobjectionable. The film comprises alternating layers of first (A)and second (B) diverse polymeric materials in which the six layersalternating repeat unit has relative optical thicknesses of about0.778A0.111B0.111A0.778B0.111A0.111B. The use of only six layers in therepeat unit results in more efficient use of material and simplermanufacture than previous designs. A repeat unit gradient may beintroduced across the thickness of the film. Thus, in one embodiment,the repeat unit thicknesses will increase linearly across the thicknessof the film. By linearly, it is meant that the repeat unit thicknessesincrease at a constant rate across the thickness of the film. In someembodiments, it may be desirable to force the repeat unit opticalthickness to double from one surface of the film to another. The ratioof repeat unit optical thicknesses can be greater or less than two aslong as the short wavelength range of the reflectance band is above 770nm and the long wavelength edge is about 2000 nm. Other repeat unitgradients may be introduced by using logarithmic and/or quarticfunctions. A logarithmic distribution of repeat unit thicknesses willprovide nearly constant reflectance across the infrared band.

In an another embodiment, the two component film may comprise a firstportion and a second portion of alternating layers. The first portionhas the six layers alternating layer repeat unit that reflects infraredlight of wave lengths between about 1200-2000 nm. The second portion ofalternating layers has an AB repeat unit, has substantially equaloptical thickness, and reflects infrared light of wavelengths betweenabout 770-1200 nm. Such a combination of alternating layers results inreflection of light across the infrared wavelength region through about2000 nm. The combination is commonly known as a “hybrid design.”Preferably, the first portion of the alternating layers has a repeatunit gradient of about 5/3:1, and the second portion of alternatinglayers have a layer thickness gradient of about 1.5:1. The hybrid designmay be provided as described for example in U.S. Pat. No. 5,360,659, buthas broader application in that it is useful with any of the broadbandinfrared reflectors or multicomponent optical designs described herein.

Another useful film design is described in U.S. patent application Ser.No. 09/006,118 filed Jan. 13, 1998 entitled “Multicomponent ReflectiveFilm,” which is incorporated herein by reference. Optical films andother optical bodies are described which exhibit a first orderreflection band for at least one polarization of electromagneticradiation in a first region of the spectrum. Such optical films suppressat least the second, and preferably also at least the third, higherorder harmonics of the first reflection band, while the percentagereflection of the first order harmonic remains essentially constant, orincreases, as a function of angle of incidence. This is accomplished byforming at least a portion of the optical body out of polymericmaterials A, B, and C which are arranged in a repeating sequence ABC,wherein A has refractive indices n_(x) ^(A), n_(y) ^(A), and n_(z) ^(A)along mutually orthogonal axes x, y, and z, respectively. Similarly,material B has refractive indices n_(x) ^(B), n_(y) ^(B), and n_(z) ^(B)along axes x, y and z, respectively, and C has refractive indices n_(x)^(C), n_(y) ^(C) and n_(z) ^(C) along axes x, y, and z, respectively.The z-axis is orthogonal to the plane of the film or optical body. Inthe optical film, n_(x) ^(A)>n_(x) ^(B)>n_(x) ^(C) or n_(y) ^(A)>n_(y)^(B)>n_(y) ^(C), and n_(z) ^(C)≧n_(z) ^(B)≧n_(z) ^(A). Preferably, atleast one of the differences n_(z) ^(A)−n_(z) ^(B) and n_(z) ^(B)−n_(z)^(C) is less than about −0.05.

By designing the film or optical body within these constraints, at leastsome combination of second, third and fourth higher-order reflectionscan be suppressed without a substantial decrease of the first harmonicreflection with angle of incidence, particularly when the firstreflection band is in the infrared region of the spectrum. Such filmsand optical bodies are particularly useful as IR mirrors, and may beused advantageously as window films and in similar applications where IRprotection is desired but good transparency and low color are important.

A modular feedblock of the type described herein, having a changeablegradient plate adaptable to vary the thickness of individual layerthicknesses or layer thickness profiles without necessitating changingor re-machining the entire feedblock assembly, is especially useful formodifying layer thickness profiles as described above.

The various layers in the film preferably have different thicknessesacross the film. This is commonly referred to as the layer thicknessgradient. A layer thickness gradient is selected to achieve the desiredband width of reflection. One common layer thickness gradient is alinear one, in which the thickness of the thickest layer pairs is acertain percent thicker than the thickness of the thinnest layer pairs.For example, a 1.055:1 layer thickness gradient means that the thickestlayer pair (adjacent to one major surface) is 5.5% thicker than thethinnest layer pair (adjacent to the opposite surface of the film). Inanother embodiment, the layer thickness could decrease, increase, anddecrease again from one major surface of the film to the other. This isbelieved to provide sharper bandedges, and thus a sharper or more abrupttransition from reflective to transmissive regions of the spectrum. Thispreferred method for achieving sharpened bandedges is described morefully in U.S. patent application Ser. No. 09/006,085 entitled “OpticalFilm with Sharpened Bandedge” filed Jan. 13, 1998, the contents of whichare herein incorporated by reference.

The method of achieving sharpened band edges will be briefly describedfor a multilayer film having layers arranged in an alternating sequenceof two optical materials, “A” and “B”. Three or more distinct opticalmaterials can be used in other embodiments. Each pair of adjacent “A”and “B” layers make up an optical repeating unit (ORU), beginning at thetop of the film with ORU1 and ending with ORU6, with the ORUs havingoptical thicknesses OT₁, OT₂, . . . OT₆. For maximum first orderreflectance (M=1 in equation I) at a design wavelength, each of the ORUsshould have a 50% f-ratio with respect to either the A or B layer. The Alayers can be considered to have a higher X- (in-plane) refractive indexthan the B layers because the former are shown to be thinner than thelatter. ORUs 1-3 may be grouped into a multilayer stack S1 in which theoptical thickness of the ORUs decrease monotonically in the minus-Zdirection, while ORUs 4-6 may be grouped into another multilayer stackS2 in which the optical thickness of the ORUs increase monotonically.Such thickness profiles are helpful in producing sharpened spectraltransitions. In contrast, thickness profiles of previously known filmstypically increase or decrease monotonically in only one direction. Ifdesired for some applications, a discontinuity in optical thickness canbe incorporated between the two stacks to give rise to a simple notchtransmission band spectrum.

Other thickness gradients may be designed which improve peaktransmission and to make even steeper band edges (narrower transmissionband). This can be achieved by arranging the individual layers intocomponent multilayer stacks where one portion of the stacks hasoppositely curved thickness profiles and the adjacent portions of thestacks have a slightly curved profile to match the curvature of thefirst portion of the stacks. The curved profile can follow any number offunctional forms. The main purpose of the form is to break the exactrepetition of thickness present in a quarter wave stack with layerstuned to only a single wavelength. The particular function used is anadditive function of a linear profile and a sinusoidal function to curvethe profile with an appropriate negative or positive first derivative.An important feature is that the second derivative of the ORU thicknessprofile be positive for the red (long wavelength) band edge of areflectance stack and negative for the blue (short wavelength) band edgeof a reflectance stack. The opposite sense is required if one refers tothe band edges of the notched transmission band. Other embodimentsincorporating the same principle include layer profiles that havemultiple points with a zero value of the first derivative. In all caseshere, the derivatives refer to those of a best fit curve fitted throughthe actual ORU optical thickness profile, which can contain smallstatistical errors of less than 10% sigma, one standard deviation inoptical thickness values.

The multilayer stack exiting the feedblock may then directly enter afinal shaping unit such as a die. Alternatively, the stream may besplit, preferably normal to the layers, to form two or more multilayerstreams that may be recombined by stacking. The stream may also be splitat an angle other than that normal to the layers. A flow channelingsystem that splits and stacks the streams is called a multiplier orinterfacial surface generator (ISG). The width of the split streams canbe equal or unequal. The multiplier ratio is defined by the ratio of thewider to narrower stream widths. Unequal streams widths (i.e.,multiplier ratios greater than unity) can be useful in creating layerthickness gradients. In the case of unequal streams, the multipliershould spread the narrower stream and/or compress the wider streamtransversely to the thickness and flow directions to ensure matchinglayer widths upon stacking. Many designs are possible, including thosedisclosed in U.S. Pat. Nos. 3,565,985; 3,759,647; 5,094,788; and5,094,793 to Schrenk et al. In typical practice, the feed to amultiplier is rectangular in cross-section, the two or more splitstreams are also rectangular in cross-section, and rectangularcross-sections are retained through the flow channels used to re-stackthe split streams. Preferably, constant cross-sectional area ismaintained along each split stream channel, though this is not required.

Each original portion of the multilayer stack that exits the feedblockmanifold, excluding PBLs, is known as a packet. In a film for opticalapplications, each packet is designed to reflect, transmit, or polarizeover a given band of wavelengths. More than one packet may be present asthe multilayer stack leaves the feedblock. Thus, the film may bedesigned to provide optical performance over dual or multiple bands.These bands may be separate and distinct, or may be overlapping.Multiple packets may be made of the same or of different combinations oftwo or more polymers. Multiple packets in which each packet is made ofthe same two or more polymers may be made by constructing the feedblockand its gradient plate in such a way that one melt train for eachpolymer feeds all packets, or each packet may be fed by a separate setof melt trains. Packets designed to confer on the film other non-opticalproperties, such as physical properties, may also be combined withoptical packets in a single multilayer feedblock stack.

An alternative to creating dual or multiple packets in the feedblock isto create them from one feedblock packet via the use of a multiplierwith multiplier ratio greater than unity. Depending on the bandwidth ofthe original packet and the multiplier ratio, the resulting packets canbe made to overlap in bandwidth or to leave between them a bandwidthgap. It will be evident to one skilled in the art that the bestcombination of feedblock and multiplier strategies for any given opticalfilm will depend on many factors, and must be determined on anindividual basis.

Prior to multiplication, additional layers can be added to themultilayer stack. These outer layers perform as PBLs, but this time,within the multiplier. After multiplication and stacking, part of thePBL streams will form internal boundary layers between optical layers,while the rest will form skin layers. Thus the packets are separated byPBLs in this case. Additional PBLs can be added and additionalmultiplication steps may be accomplished prior to final feed into aforming unit such as a die. Prior to the final feed, additional layerscan be added to the outside of the multilayer stack, whether or notmultiplication has been performed, and whether or not PBLs have beenadded prior to the multiplication step. The additional layers form thefinal skin layers and the external portions of the earlier-applied PBLswill form sub-skins under these final skin layers. The die performs theadditional compression and width spreading of the melt stream. Again,the die (including its internal manifold, pressure zones, etc.) isdesigned to create uniformity of the layer distribution across the webas it exits the die.

Skin layers are frequently added to the multilayer stack to protect thethinner optical layers from the effects of wall stress and possibleresulting flow instabilities. Other reasons for adding a thick layer atthe surface(s) of the film include, e.g., surface properties such asadhesion, coatability, release, coefficient of friction, and barrierproperties, weatherability, scratch and abrasion resistance, and others.In multilayer films that are subsequently uniaxially or very unequallybiaxially drawn, “splittiness,” (i.e., the tendency to tear or faileasily along the more highly drawn direction), can be substantiallysuppressed by choosing a skin layer polymer that (1) adheres well to thesub-skin or nearest optical layer polymer and (2) is less prone toorientation upon draw. An example of a useful skin layer, where theoptical stack contains a PEN homopolyer, is a copolymer of PEN having acomonomer content sufficient to suppress crystallinity and/orcrystalline orientation. Marked suppression of splittiness is observedin such a structure, compared to a similar film without the coPEN skinlayer(s), when the films are highly drawn in one planar direction andundrawn or only slightly drawn in the orthogonal planar direction. Oneskilled in the art will be able to select similar skin layer polymers tocomplement other optical layer polymers and/or sub-skin polymers.

Temperature control is important in the feedblock and subsequent flowleading to casting at the die lip. While temperature uniformity is oftendesired, in some cases, deliberate temperature gradients in thefeedblock or temperature differences of up to about 40° C. in the feedstreams can be used to narrow or widen the stack layer thicknessdistribution. Feed streams into the PBL or skin blocks can also be setat different temperatures than the feedblock average temperature. Often,the PBL or skin streams are about 40° C. higher than the feed streamtemperature to reduce viscosity or elasticity in the protective streamsand thus enhance their effectiveness as protective layers. Sometimes,the protective streams' temperature can be decreased up to about 40° C.to improve the rheology matching between them and the rest of the flowstream. For example, decreasing the temperature of a low viscosity skinmay enhance viscosity matching and enhance flow stability. Other times,elastic effects need to be matched.

Conventional means for heating the feedblock-multiplier-die assembly,namely, the use of insertion- or rod- or cartridge-type heaters fittedinto bores in the assembly, are frequently incapable of providing thetemperature control required for the inventive optical films.Preferably, heat is provided uniformly from outside the assembly by (i)tiling its exterior with plate-type heaters, (ii) insulating thoroughlythe entire assembly, or (iii) combining the two techniques. Plate-typeheaters typically use a resistance-heating element embedded in a metalmaterial, such as cast aluminum. Such heaters can distribute heatuniformly to an apparatus, such as, e.g., the feedblock.

The use of insulation to control heat flow is not new. It is, however,typically not done in film extrusion due to the possibility of polymermelt leakage from the assembly onto the insulation. Because of the needto regulate layer flows very precisely, such leakage cannot be toleratedin the feedblock-multiplier-die assemblies used for the inventiveoptical films. Thus, feedblocks, multipliers, and dies are carefullydesigned, machined, assembled, connected, and maintained so as toprevent polymer melt leakage, and insulation of the assembly becomesboth feasible and preferred.

An insertion- or rod- or cartridge-type heater, having both a specificdesign and specific placement within the feedblock, is advantageous bothfor maintaining constant temperature in the feedblock and for creating atemperature gradient of up to about 40° C. This heater, called an axialrod heater, consists of a heater placed in a bore through the feedblockand oriented in a direction normal to the layer plane, preferably verynear an imaginary line through the points where each side channel tubefeeds a slot die. More preferably, in the case of coextrusion of a firstpolymer and a second polymer, the bore for the axial rod heater will belocated both near an imaginary line through the points where each sidechannel tube feeds a slot die, and also equidistant from the sidechannel tubes carrying the first polymer and the side channel tubescarrying the second polymer. Further, the axial rod heater is preferablyof a type that can provide a temperature gradient or a multiplicity ofdiscrete temperatures along its length, either by variation inelectrical resistance along its length, or by multi-zone control, or byother means known in the art. Such a heater, used in conjunction withthe plate-type heaters described above, the insulation described above,or both, provides superior temperature control and/or uniformity totraditional means. Such superior control over layer thickness andgradient layer thickness distribution is especially important incontrolling the positions and profiles of reflection bands as describedin U.S. patent applications Ser. No. 09/006,085 entitled “Optical Filmwith Sharpened Bandedge” and Ser. No. 09/006,591 entitled “ColorShifting Film,” both filed Jan. 13, 1998 and the contents of which areincorporated herein by reference.

Shear rate is observed to affect viscosity and other rheologicalproperties, such as elasticity. Flow stability sometimes appears toimprove by matching the relative shape of the viscosity (or otherTheological function) versus shear rate curves of the coextrudedpolymers. In other words, minimization of maximal mismatch between suchcurves may be an appropriate objective for flow stability. Thus,temperature differences at various stages in the flow can help tobalance shear or other flow rate differences over the course of thatflow.

The web is cast onto casting roll, sometimes referred to as a castingwheel or casting drum. The casting roll is preferably chilled to quenchthe web and begin the formation of a multilayer cast film. Preferably,casting is assisted by electrostatic pinning, the details of which arewell-known in the art of polyester film manufacture. For the inventiveoptical films, care should be exercised in setting the parameters of theelectrostatic pinning apparatus. Periodic cast web thickness variationsalong the extrusion direction of the film, frequently referred to as“pinning chatter,” should be avoided to the extent possible. Adjustmentsto the current, voltage, pinning wire thickness, and pinning wirelocation with respect to the die and the casting chill roll are allknown to have an affect, and should be set on a case-by case basis byone skilled in the art.

The web can sometimes attain a sidedness in surface texture, degree ofcrystallinity, or other properties due to wheel contact on one side andmerely air contact on the other. This can be desirable in someapplications and undesirable in others. When minimization of suchsidedness differences is desired, a nip roll can be used in combinationwith the casting roll to enhance quenching or to provide smoothing ontowhat would otherwise be the air side of the cast web.

In some cases, it is important that one side of the multilayer stack bethe side chosen for the superior quench that is attained on the chillroll side. For example, if the multilayer stack consists of adistribution of layer thicknesses, it is frequently desired to place thethinnest layers nearest the chill roll. This is discussed in detail inU.S. patent application Ser. No. 08/904,325, entitled “Method for MakingOptical Films Having Thin Optical Layers,” which is incorporated hereinby reference.

In some cases, it is desired to provide the film with a surfaceroughness or surface texture to improve handling in winding and/orsubsequent conversion and use. A specific example germane to theinventive optical films arises when they are intended for use inintimate contact with a glass plate or a second film. In such cases,selective “wetting out” of the optical film onto the plate or secondfilm can result in the phenomenon known as “Newton's Rings,” whichdamages the uniformity of the optics over large surface areas. Atextured or rough surface prevents the intimacy of contact required forwetting out thereby minimizing or eliminating the appearance of Newton'sRings.

It is well known in the polyester film art to include small amounts offine particulate materials, often referred to as “slip agents,” toprovide such surface roughness or texture. The use of slip agents can beincorporated into the inventive optical films. However, the inclusion ofslip agent particulates can introduce a small amount of haze and candecrease the optical transmission of the film. In accordance with thepresent invention, Newton's Rings can be effectively prevented, withoutthe use of slip agents, if surface roughness or texture is provided bycontacting the cast web with a micro-embossing roll during film casting.Preferably, the micro-embossing roll will serve as a nip roll to thecasting wheel. Alternatively, the casting wheel itself may bemicro-textured to provide a similar effect. Further, both amicro-embossing casting wheel and a micro-embossing nip roll may be usedtogether to provide a film that is micro-embossed on both sides.

Further, Applicants found that the use of a smooth nip roll at thecasting roll, in addition to aiding quench at what would otherwise bethe air side of the film, as already discussed above, can alsosignificantly reduce the magnitude of die lines, pinning chatter, andother thickness fluctuations. The web may be cast to a uniform thicknessacross the web or a deliberate profiling of the web thickness may beinduced using die lip controls. Such profiles may improve uniformity bythe end of the film process. In other cases, a uniform cast thicknessprovides best uniformity at the end of the film process. Controllingvibrations in the process equipment is also important to reduce“chatter” in the cast multilayer web.

Residence times in the various process stages may also be important evenat a fixed shear rate. For example, interdiffusion between layers can bealtered and controlled by adjusting residence times. “Interdiffusion,”as used in this document, refers to mingling and reactive processesbetween materials of the individual layers including, for example,various molecular motions such as normal diffusion, cross-linkingreactions, or transesterification reactions. Sufficient interdiffusionis desirable to ensure good interlayer adhesion and preventdelamination. However, too much interdiffusion can lead to deleteriouseffects, such as the substantial loss of compositional distinctnessbetween layers. Interdiffusion can also result in copolymerization ormixing between layers, which may reduce the ability of a layer to beoriented when drawn. The scale of residence time on which suchdeleterious interdiffusion occurs is often much larger (e.g., by anorder of magnitude) than that required to achieve good interlayeradhesion, thus the residence time can be optimized. However, somelarge-scale interdiffusion may be useful in profiling the interlayercompositions, for example to make rugate structures.

The effects of interdiffusion can also be altered by further layercompression. Thus, the effect at a given residence time is also afunction of the state of layer compression during that interval relativeto the final layer compression ratio. As thinner layers are moresusceptible to interdiffusion, they are typically placed closest to thecasting wheel for maximal quenching.

Applicants also found that interdiffusion can be enhanced after themultilayer film has been cast, quenched, and drawn, via heat setting atan elevated temperature. Heat setting is normally done in the tenteroven in a zone subsequent to the transverse drawing zone. Normally, forpolyester films, the heat setting temperature is chosen to maximizecrystallization rate and optimize dimensional stability properties. Thistemperature is normally chosen to be between the glass transition andmelting temperatures, and not very near either temperature. Selection ofa heat set temperature closer to the melting point of the lowest-meltingpolymer among those polymers in the multilayer film which are desired tomaintain orientation in the final state results in a marked improvementin interlayer adhesion. This is unexpected due to the short residencetimes involved in heat setting on line, and the non-molten nature of thepolymers at this process stage. Further, while offline heat treatmentsof much longer duration are known to improve interlayer adhesion inmultilayer films, these treatments also tend to degrade otherproperties, such as modulus or film flatness, which was not observedwith on-line elevated-temperature heat setting treatments.

Conditions at the casting wheel are set according to the desired result.Quenching temperatures must be cold enough to limit haze when opticalclarity is desired. For polyesters, typical casting temperatures rangebetween 10° C. and 60° C. The higher portion of the range may be used inconjunction with smoothing or embossing rolls while the lower portionleads to more effective quenching of thick webs. The speed of thecasting wheel may also be used to control quench and layer thickness.For example, the extruder pumping rates may be slowed to reduce shearrates or increase interdiffusion while the casting wheel is increased inspeed to maintain the desired cast web thickness. The cast web thicknessis chosen so that the final layer thickness distribution covers thedesired spectral band at the end of all drawing with concomitantthickness reductions.

The multilayer web is drawn to produce the final multilayer opticalfilm. A principal reason for drawing is to increase the optical power ofthe final optical stack by inducing birefringence in one or more of thematerial layers. Typically, at least one material becomes birefringentunder draw. This birefringence results from the molecular orientation ofthe material under the chosen draw process. Often this birefringencegreatly increases with the nucleation and growth of crystals induced bythe stress or strain of the draw process (e.g. stress-inducedcrystallization). Crystallinity suppresses the molecular relaxation,which would inhibit the development of birefringence, and crystals maythemselves also orient with the draw. Sometimes, some or all of thecrystals may be pre-existing or induced by casting or preheating priorto draw. Other reasons to draw the optical film may include, but are notlimited to, increasing throughput and improving the mechanicalproperties in the film.

In one typical method for making a multilayer optical polarizer, asingle drawing step is used. This process may be performed in a tenteror a length orienter. Typical tenters draw transversely (TD) to the webpath, although certain tenters are equipped with mechanisms to draw orrelax (shrink) the film dimensionally in the web path or machinedirection (MD). Thus, in this typical method, a film is drawn in onein-plane direction. The second in-plane dimension is either heldconstant as in a conventional tenter, or is allowed to neck into asmaller width as in a length orienter. Such necking in may besubstantial and increases with draw ratio. For an elastic,incompressible web, the final width may be estimated theoretically asthe reciprocal of the square root of the lengthwise draw ratio times theinitial width. In this theoretical case, the thickness also decreases bythis same proportion. In practice, such necking may produce somewhatwider than theoretical widths, in which case the thickness of the webmay decrease to maintain approximate volume conservation. However,because volume is not necessarily conserved, deviations from thisdescription are possible.

In one typical method for making a multilayer mirror, a two step drawingprocess is used to orient the birefringent material in both in-planedirections. The draw processes may be any combination of the single stepprocesses described that allow drawing in two in-plane directions. Inaddition, a tenter that allows drawing along MD, e.g. a biaxial tenter,which can draw in two directions sequentially or simultaneously, may beused. In this latter case, a single biaxial draw process may be used.

In still another method for making a multilayer polarizer, a multipledrawing process is used that exploits the different behavior of thevarious materials to the individual drawing steps to make the differentlayers comprising the different materials within a single coextrudedmultilayer film possess different degrees and types of orientationrelative to each other. Mirrors can also be formed in this manner. Suchoptical films and processes are described further in U.S. patentapplication Ser. No. 09/006,455 filed Jan. 13, 1998 entitled “An OpticalFilm and Process for Manufacture Thereof,” the contents of which isincorporated by reference.

Drawing conditions for multilayer optical polarizer films are oftenchosen so that a first material becomes highly birefringent in-planeafter draw. A birefringent material may be used as the second material.If the second material has the same sense of birefringence as the first(e.g. both materials are positively birefringent), then it is usuallypreferred to choose the second material so that is remains essentiallyisotropic. In other embodiments, the second material is chosen with abirefringence opposite in sense to the first material when drawn (e.g.,if the first material is positively birefringent, the second material isnegatively birefringent). For a positively birefringent first material,the direction of highest in-plane refractive index, the first in-planedirection, coincides with the draw direction, while the direction oflowest in-plane refractive index for the first material, the secondin-plane direction, is perpendicular to the first direction. Similarly,for multilayer mirror films, a first material is chosen to have largeout-of-plane birefringence, so that the in-plane refractive indices areboth higher than the initial isotropic value in the case of a positivelybirefringent material (or lower in the case of a negatively birefringentmaterial). In the mirror case, it is often preferred that the in-planebirefringence is small so that the reflections are similar for bothpolarization states, i.e. a balanced mirror. The second material for themirror case is then chosen to be isotropic, or birefringent in theopposite sense, in similar fashion to the polarizer case.

In another embodiment of multilayer optical films, polarizers may bemade via a biaxial process. In still another embodiment, balancedmirrors may be made by a process that creates two or more materials ofsignificant in-plane birefringence and thus in-plane asymmetry such thatthe asymmetries match to form a balanced result, e.g. nearly equalrefractive index differences in both principal in-plane directions.

In certain processes, rotation of these axes can occur due to theeffects of process conditions including tension changes down web. Thisis sometimes referred to as “bowforward” or “bow-back” in film made onconventional tenters. Uniform directionality of the optical axes isusually desirable for enhanced yield and performance. Processes thatlimit such bowing and rotation, such as tension control or isolation viamechanical or thermal methods, can be used.

Frequently, it is observed that drawing film transverse to the machinedirection in a tenter is non-uniform, with thickness, orientation, orboth changing as the film approaches the gripped edges of the web.Typically, these changes are consistent with the assumption of a coolerweb temperature near the gripped edges than in the web center. Theresult of such non-uniformity can be a serious reduction in usable widthof the finished film. This restriction can be even more severe for theoptical films of the present invention, as very small differences infilm thickness can result in non-uniformity of optical properties acrossthe web. Drawing, thickness, and color uniformity, as Applicantsrecognize, can be improved by the use of infrared heaters to heatfurther the edges of the film web near the tenter grippers. Suchinfrared heaters can be used before the tenter's preheat zone, in thepreheat zone, in the stretch zone, or in a combination of locations. Oneskilled in the art will appreciate the many options for zoning andcontrolling the addition of infrared heat. Further, the possibilitiesfor combining infrared edge heating with changes in the cast web'scross-web thickness profile will also be apparent.

For certain of the inventive multilayer optical films, it is desirableto draw the film in such a way that one or more properties, measured onthe finished films, have identical values in the machine and transversedirections. Such films are often referred to as “balanced” films.Machine- and transverse-direction balance can be achieved by selectingprocess conditions using techniques well known in the art of biaxiallyoriented film making. Typically, process parameters explored includemachine-direction orientation preheat temperature, stretch temperature,and draw ratio, tenter preheat temperature, tenter stretch temperature,and tenter draw ratio, and, sometimes, parameters related to thepost-stretching zones of the tenter. Other parameters may also besignificant. Typically, designed experiments are performed and analyzedto arrive at appropriate combinations of conditions. Those skilled inthe art will appreciate the need to perform such an assessmentindividually for each film construction and each film line on which itis to be made.

Similarly, parameters of dimensional stability (such as shrinkage atelevated temperature and reversible coefficient of thermal expansion)are affected by a variety of process conditions. Such parametersinclude, but are not limited to, heat set temperature, heat setduration, transverse direction dimensional relaxation (“toe-in”) duringheat set, web cooling, web tension, and heat “soaking” (or annealing)after winding into rolls. Again, designed experiments can be performedby one skilled in the art to determine optimum conditions for a givenset of dimensional stability requirements, for a given film composition,and for a given film line.

In general, multilayer flow stability is achieved by matching orbalancing the Theological properties, such as viscosity and elasticity,between the first and second materials to within a certain tolerance.The level of required tolerance or balance also depends on the materialsselected for the PBL and skin layers. In many cases, it is desirable touse one or more of the optical stack materials individually in thevarious PBL or skin layers. For polyesters, the typical ratio betweenhigh and low viscosity materials is no more than 4:1, preferably no morethan 2:1, and most preferably no more than 1.5:1 for the processconditions typical of feedblocks, multipliers, and dies. Using the lowerviscosity optical stack material in the PBL and skin layers usuallyenhances flow stability. More latitude in the requirements for a secondmaterial to be used with a given first material is often gained bychoosing additional materials for the PBL and skin layers. Often, theviscosity requirements of these third materials (PBL and skin layers)are then balanced with the effective average viscosities of themultilayer stack comprising the first and second materials. Typically,the viscosity of the PBL and skin layers should be lower than this stackaverage for maximal stability. If the process window of stability islarge, higher viscosity materials can be used in these additionallayers, for example, to prevent sticking to rollers downstream ofcasting in a length orienter.

Draw compatibility means that the second material can undergo the drawprocessing needed to achieve the desired birefringence in the firstmaterial without causing deleterious effects to the multilayer film,such as breakage, voiding, or stress whitening. These effects can causeundesired optical properties. Draw compatibility usually requires thatthe glass transition temperature of the second material be no more thanabout 40° C. higher than that of the first material. This limitation canbe ameliorated (1) by very fast drawing rates that make the orientationprocess for the first material effective even at higher temperatures or(2) by crystallization or cross-linking phenomena that also enhance theorientation of the first material at such higher temperatures. Also,draw compatibility requires that the second material can achieve thedesired optical state at the end of processing, whether this is anessentially isotropic state or a highly birefringent state.

In the case of a second material that is to remain isotropic after finalprocessing, at least three methods of material selection and processingcan be used to meet this second requirement for draw compatibility.First, the second material can be inherently non-birefringent. Anexample of an inherently non-birefringent material is polymethylmethacrylate because it remains optically isotropic (as measuredby refractive index) even if there is substantial molecular orientationafter drawing. Second, the second material can be chosen so as to remainunoriented at the draw conditions of the first material, even though itcould be made birefringent if drawn under different conditions. Third,the second material can orient during the draw process provided it maylose the orientation so gained in a subsequent process, such as aheat-setting step. In the case of multiple drawing schemes in which thefinal desired film contains more than one highly birefringent material(e.g. a polarizer made in certain biaxial drawing schemes), drawcompatibility may not require any of these methods. Alternatively, thethird method may be applied to achieve isotropy after a given drawingstep, or any of these methods may be used for third or furthermaterials.

Draw conditions can also be chosen to take advantage of the differentvisco-elastic characteristics of the first and second optical materials,as well as any materials used in the skin and PBL layers, such that thefirst material becomes highly oriented during draw while the secondremains unoriented or only slightly oriented after draw according to thesecond scheme described above. Visco-elasticity is a fundamentalcharacteristic of polymers. The visco-elasticity characteristics of apolymer may be used to describe its tendency to react to strain like aviscous liquid or an elastic solid. At high temperatures and/or lowstrain rates, polymers tend to flow when drawn like a viscous liquidwith little or no molecular orientation. At low temperatures and/or highstrain rates, polymers tend to draw elastically like solids withconcomitant molecular orientation. A low temperature process istypically considered take place near the polymeric material's glasstransition temperature, while a high temperature process takes placesubstantially above the glass temperature.

Visco-elastic behavior is generally the result of the rate of molecularrelaxation in a polymeric material. In general, molecular relaxation isthe result of numerous molecular mechanisms, many of which are molecularweight dependent. Thus, polydisperse polymeric materials have adistribution of relaxation times, with each molecular weight fraction inthe polydisperse polymer having its own longest relaxation time. Therate of molecular relaxation can be characterized by an average longestoverall relaxation time (i.e., overall molecular rearrangement) or adistribution of such times. The precise numerical value for the averagelongest relaxation time for a given distribution is a function of howthe various times in the distribution are weighted in the average. Theaverage longest relaxation time typically increases with decreasingtemperature and becomes very large near the glass transitiontemperature. The average longest relaxation time can also be increasedby crystallization and/or crosslinking in the polymeric material which,for practical purposes, inhibits any relaxation under process times andtemperatures typically used. Molecular weight and distribution, as wellas chemical composition and structure (e.g., branching), can also effectthe longest relaxation time.

The choice of resin strongly effects the characteristic relaxation time.Average molecular weight, MW, is a particularly significant factor. Fora given composition, the characteristic time tends to increase as afunction of molecular weight (typically as the 3 to 3.5 power ofmolecular weight) for polymers whose molecular weight is well above theentanglement threshold. For unentangled polymers, the characteristictime tends to increase as a weaker function of molecular weight. Becausepolymers below this threshold tend to be brittle when below their glasstransition temperatures and are usually undesirable, they are not theprincipal focus here. However, certain lower molecular materials may beused in combination with layers of higher molecular weight as could lowmolecular weight rubbery materials above the glass transition, e.g. anelastomeric or tacky layer. Inherent or intrinsic viscosity, IV, ratherthan average molecular weight, is usually measured in practice. The IVvaries as MW^(α) where α is the solvent dependent Mark-Houwink exponent.The exponent α increases with solubility of the polymer. Typical valuesof α might be 0.62 for PEN (polyethylene naphthalate) and 0.68 for PET(polyethylene terephthalate), both measured in solutions of 60:40Phenol:ortho-Dichlorobenzene, with intermediate values for a copolymerof the two (e.g., coPEN). PBT (polybutylene terephthalate) would beexpected to have a still larger value of α than PET, as would polyestersof longer alkane glycols (e.g. hexane diol) assuming improved solubilityin the chosen solvent. For a given polymer, better solvents would havehigher exponents than those quoted here. Thus, the characteristic timeis expected to vary as a power law with IV, with its power exponentbetween 3/α and 3.5/α. For example, a 20% increase in IV of a PEN resinis expected to increase the effective characteristic time. Thus theWeissenberg Number (as defined below) and the effective strength of thedrawing flow, at a given process temperature and strain rate by a factorof approximately 2.4 to 2.8. Since a lower IV resin will experience aweaker flow, relatively lower IV resins are preferred in the presentinvention for the case of a second polymer of desired low finalbirefringence, and higher IV resins are preferable for the strongerflows required of the first polymer of high birefringence. The limits ofpractice are determined by brittleness on the low IV end and by the needto have adequate rheological compatibility during the coextrusion. Inother embodiments, in which strong flows and high birefringence aredesired in both a first and second material, higher IV may be desiredfor both materials. Other processing considerations, such as upstreampressure drops as might be found in the melt stream filters, can alsobecome important.

The severity of a strain rate profile can be characterized in a firstapproximation by a Weissenberg number (Ws) which is the product of thestrain rate and the average longest relaxation time for a givenmaterial. The threshold Ws value between weak and strong draw (belowwhich, and above which, the material remains isotropic or experiencesstrong orientation, crystallization and high birefringence,respectively) depends on the exact definition of this average longestrelaxation time as an average of the longest relaxation times in thepolydisperse polymeric material. It will be appreciated that theresponse of a given material can be altered by controlling the drawingtemperature, rate and ratio of the process. A process which occurs in ashort enough time and/or at a cold enough temperature to inducesubstantial molecular orientation is an orienting or strong drawprocess. A process which occurs over a long enough period and/or at hotenough temperatures such that little or no molecular orientation occursis a non-orienting or weak process.

Another critical issue is the duration of the draw process. Strong drawprocesses typically need enough duration (that is, a high enough drawratio) to accomplish sufficient orientation, e.g. to exceed thethreshold for strain-induced crystallization, thereby achieving highbirefringence in the first material. Thus, the strain rate historyprofile, which is the collection of the instantaneous strain rates overthe course of the drawing sequence, is a key element of the drawprocess. The accumulation of the instantaneous strain rates over theentire draw process determines the final draw ratio. The temperature andstrain rate draw profile history determine the draw ratio at which thefirst polymer experiences the onset of strain-induced crystallization,given the characteristic time and supercooling of that polymer.Typically, the onset draw ratio decreases with increasing Ws. For PET,experimental evidence suggests this onset draw ratio has a limit between1.5 and 2 at very high rates of strain. At lower rates of strain, theonset draw ratio for PET can be over 3. The final level of orientationoften correlates with the ratio of the final draw ratio to the onsetdraw ratio.

Temperature has a major effect on the characteristic average longestrelaxation time of the material, and is thus a major factor indetermining whether a given material experiences a weak or strong flow.The dependence of the characteristic time on temperature can bequantified by the well known WLF equation [See. J. D. Ferry,Viscoelastic Properties of Polymers, John Wiley & Sons, New York, 1970].This equation contains three parameters, c₁, c₂ and T₀. Often, T₀ isassociated with the glass transition temperature, T_(g). Using theapproximate “universal” values for c₁ and c₂, applicable as a firstestimate for many polymers, the WLF equation shows the large dependenceon relaxation times with temperature. For example, using a relaxationtime at 5° C. higher than the T_(g) as a value for comparison, therelaxation times at 10° C., 15° C., and 20° C. higher than T_(g) areapproximately 20, 250 and 2000 times shorter, respectively. Greateraccuracy for WLF parameters can be obtained by using empirical curvefitting techniques for a particular class of polymers, e.g. polyesters.Thus, to a first approximation, the single most important parameter fortemperature effects on the characteristic time is T_(g). The larger thetemperature difference between the web temperature and T_(g), thesmaller the characteristic time and thus the weaker the draw flow.Further, it is reiterated that this discussion is most pertinent to thedraw process prior to crystallization, especially strain inducedcrystallization. After crystallization occurs, the presence of crystalscan further retard relaxation times and convert otherwise weak flows tostrong flows.

By selecting the materials and process conditions in consideration ofthe orienting/non-orienting response of the materials, a film can beconstructed such that the first material is oriented and birefringentand the second material is essentially unoriented. That is, the processis a strong draw process for the first material and a weak draw processfor the second material. As an example of strong and weak flows, let usconsider PEN of approximately 0.48 IV, an initial draw rate of about 15%per second, and a uniaxial draw profile that increases the draw ratio ina linear manner to a final draw ratio of 6.0. At a web temperature ofabout 155° C., PEN experiences weak flow that leaves it in a state oflow birefringence. At 135° C., PEN experiences a strong flow that makesit highly birefringent. The degree of orientation and crystallizationincreases in this strong flow regime as the temperature drops further.These values are for illustration only and should not be taken as thelimiting values of these regimes.

More general ranges for material selection can be understood byconsidering the more general case of polyesters. For PET, approximatevalues for the WLF parameters can be taken as c₁=11.5, c₂=55.2 andT₀=T_(g)+4° C.=80° C. These values are for purposes of illustrationonly, it being understood that empirical determination of theseconstants may give somewhat varying results. For example, alternatevalues using the “universal” values of c₁=17.7 and c₂=51.6, and usingT₀=85° C., have been proposed. At a temperature 20° C. above the glasstransition, the effect of a 5° C. increase/decrease in temperature is todecrease/increase the characteristic time and Ws by a factor of four. At10° C. above the glass transition, the effect is much stronger, about afactor of ten. For PEN, T₀ is estimated as approximately 127° C. ForDMI-based polyester (e.g. PEI), T₀ is estimated as about 64° C. Theglass transition of polyester with some higher alkane glycol such ashexane diol might be expected, based on these example WLF values, tohave a 1° C. decrease in glass transition for every 1% replacement ofethylene glycol. For coPEN, the glass transition can be estimated usingthe so-called Fox equation. The reciprocal of the coPEN glass transitiontemperature (in absolute degrees) is equal to the linear,compositionally weighted average of its component reciprocal glasstransition temperatures (in absolute degrees). Therefore, a coPEN of 70%naphthalene dicarboxylate (NDC) and 30% dimethylterephthalate (DMT)would have an estimated glass transition of about 107° C., assumingglass transitions for PEN and PET of 123° C. and 76° C., respectively.Likewise, a coPEN of 70% NDC and 30% DMI would have a glass transitionof about 102° C. Roughly, the latter coPEN would be expected toexperience a weak flow at a temperature 20° C. lower than that requiredfor weak flow for PEN, under the same conditions. Thus, at webtemperatures of 135° C., coPEN is weakly oriented and PEN is stronglyoriented under the process conditions cited. This particular choice ofresins has been previously cited as one example of a preferredembodiment for multilayer reflective polarizers in WO 95/17303.

The temperature effects the strength of the flow secondarily by alteringthe rate of nucleation and crystal growth. In the undrawn state, thereis a temperature of maximum crystallization rate. Rates are slowed belowthis temperature due to much slower molecular motions as characterizedby the relaxation times. Above this temperature, the rates are slowed bythe decrease in the degree of supercooling (the melting temperatureminus the process temperature), which is related to the thermodynamicdriving force for crystallization. If the draw is fast and thetemperature is near T_(g), the onset of strain induced crystallizationmay be enhanced (making the draw still stronger) by raising thetemperature, because little additional relaxation occurs at the highertemperature but nucleation and growth can be accelerated. If thetemperature of draw is near the melting point, raising the drawtemperature and thus decreasing the degree of supercooling may decreasethe rate of strain-induced crystallization, delaying the onset of suchcrystallization and thereby making the flow effectively weaker. Amaterial can be deliberately designed to have a low melting point andthus little or no supercooling. Copolymers are known to have a reducedmelting point due to the impurity effect of the additional monomer. Thiscan be used effectively to maintain the second polymer in a state of loworientation.

The aforementioned effect of melting point can also be used toaccomplish the third method for obtaining draw compatibility in the caseof a second material with desired isotropy. Alternatively, this may beused after a drawing step during a multiple drawing process to achieveisotropy in one or more of the materials. Drawing processes that arestrong for both the first and second material may be used as long as theeffects of that draw can be eliminated in the second polymer in asubsequent step. For example, a heat setting step can be used toaccomplish relaxation of an oriented, but still amorphous, secondpolymer. Likewise, a heat setting step can be used to melt an orientedand crystallized second polymer, as long as it is adequately quenched.

Heat setting can also be useful in improving other properties, such asdimensional stability (with regard to both temperature and humidity) andinterlayer adhesion. Finally, tension conditions at quenching, prior towinding, can also affect physical properties, such as shrinkage. Reducedwinding tension and reduced cross web tension via a toe in (reduction intransverse draw ratio) can reduce shrinkage in a variety of multilayeroptical films. Post-winding heat treatment of film rolls can also beused to improve dimensional stability and reduce shrinkage.

In general, the birefringence of a polymer experiencing a strong flowdeformation tends to increase with the draw ratio. Because ofstrain-induced crystallization, for a given draw process there may be acritical draw ratio at which this birefringence begins to increase moredramatically. After onset of crystallization, the slope may again change(e.g. drop) due to changes in the relative amount of continuednucleation and growth with further drawing. For the inventive multilayeroptical films, the increase in the birefringence of at least one of thepolymers leads to an increase in the reflection of light of wavelengthsappropriate to the layer thicknesses of the multilayer stack. Thisreflective power also tends to increase in relative measure to theorientation.

On the other hand, adhesion between layers in the multilayer stack isoften adversely affected by drawing, with stretched films frequentlybeing much more prone to exfoliation of layers than the cast webs fromwhich they were made. Surprisingly, this decrease in interlayeradhesion, as discovered by the present inventors, may also experience acritical point under some process/material combinations so that themajority of the decrease happens relatively abruptly as a specific drawratio is exceeded. This critical change need not correlate with changesin the birefringence. In other cases, the behavior can be non-linear butnot necessarily abrupt. The existence and value of this critical drawratio is likely a complex function of the polymers involved and a hostof other process conditions, and needs to be determined on acase-by-case basis. The compromise between high optical extinction andhigh interlayer adhesion with respect to draw ratio will be dominated bythe existence and location of an abrupt transition or other functionalform, e.g., with the optimal draw ratio for a given film likely to beselected from the maximum possible draw ratio and the draw ratio justbelow the abrupt interlayer adhesion transition.

There are other process compromises that may be apparent for particularresin system choices. For instance, in certain systems, higher drawratio may also result in higher off-angle color. Increased off-anglecolor can result from an increase in the z-index (the out-of-planeindex) interlayer mismatch due to the lowering of the z-index ofrefraction of the first material (such as PEN), while the secondmaterial z-index remains nearly constant. The drop in z-indices inaromatic polyesters may be related to the planarization of the crystalswithin the film, which causes the planes of the aromatic rings to tendto lie in the plane of the film. Such compromises may sometimes beavoided by altering the selection of resin pairs. For example, reducingthe level of crystallinity while maintaining a given level oforientation may improve both interlayer adhesion and off-angle colorwithout reducing extinction power, as long as the difference between therefractive index of the in-plane draw direction and the in-planenon-drawn direction remains about the same. This latter condition can bemet by using high NDC content coPENs as the first polymer. The lowermelting points of these polymers suggest that lower levels ofcrystallinity would be obtained at the same level of orientation,allowing extinction to be maintained while decreasing off-angle colorand possibly increasing interlayer adhesion. It will be appreciated thatsimilar process considerations would pertain to additional materials,such as those to be used in the skin and/or PBLs. If these materials areto be isotropic, thus avoiding polarization retardation from thickbirefringent layers, they should be chosen in accord with therequirements of a second polymer with desired isotropy.

Finally, the need for careful control and uniformity of processconditions should be appreciated to form high quality optical films inaccordance with the present invention. Draw uniformity is stronglyinfluenced by temperature, and thus uniform temperature is typicallydesired for a uniform film. Likewise, caliper (thickness) andcompositional uniformity is also desirable. One preferred method toobtain uniformity is to cast a flat uniform film, which is thenuniformly drawn to make a uniform final film. Often, final filmproperties are more uniform (in off-angle color, for example) and better(e.g. interlayer adhesion) under such processes. Under certaincircumstances, cast thickness profiling can be used to compensate foruneven drawing to produce a final film of uniform caliper. In addition,infrared edge heating, discussed above, can be used in conjunction withcast thickness profiling.

Film Uniformity

The high quality multilayer optical films and other optical devices madein accordance with the present invention can be made so as to exhibit adegree of physical and optical uniformity over a large area that farexceeds that accessible with prior art films. In accordance with themethod of the invention, the distortions of layer thickness and opticalcaliper encountered in prior art cast (not drawn) films is avoided bybiaxially stretching the cast web by a factor of between about 2×2 andabout 6×6, and preferably about 4×4. These ranges tend to make thelateral layer thickness variations, and therefore the color variations,much less abrupt. Furthermore, because the film is made by stretching acast web (as opposed to casting a finished film directly withoutstretching), the narrower the cast web width, the fewer the distortionsin layer thickness distribution in the extrusion die because ofsignificantly less layer spreading occurring in the narrower die.

Many other process considerations discussed in the sections above andintended to improve layer thickness uniformity also improve the coloruniformity, as color depends directly on layer thickness. These include,but are not limited to, multilayer resin system rheological matching,filtration, feedblock design, multiplier design, die design, PBL andskin layer selection, temperature control, electrostatic pinningparameters, use of web thickness variation scanning devices, use of acasting nip roll, vibration control, and web edge heating in the tenter.

Errors in extrusion equipment design and machining, and in the extrusioncontrol, will lead to both systematic and random thickness errors. Foruniform color films in general, the random errors can lead to both downweb and cross web variations in color, and the systematic errors,although not changing, will affect both the overall color of the filmand the crossweb color variation.

Both random and systematic errors can occur for the overall film caliperas well as for individual layers. Overall film caliper errors are mosteasily detected and monitored via the optical transmission orreflectance spectra. Thus, an on-line spectrophotometer can be set up tomeasure the spectral transmission of the film as it comes off the line,thereby providing the necessary information to measure color uniformityand provide feedback for process controls. Individual layer errors mayor may not affect the perceived color, depending mostly on where theyare in the optical stack and on the magnitude of the errors.

Systematic errors are repeatable deviations from the design thicknessfor any or all layers in the stack. They can occur because of designapproximations inherent in the polymer flow model used to design themultipliers and feedblock, or because of machining errors in thefeedblock and die. These errors can be eliminated by redesign andre-machining until the errors are reduced to design criteria. Theseerrors can also be reduced by machining a feedblock that will producethe required number of layers in the optical film without resort to amultiplier.

Random errors can be caused by: (1) fluctuations in feedblock and diezone temperatures, (2) resin non-homogeneity, (3) improper control ofmelt temperatures through the melt train, which selectively degradeparts of the melt stream, (4) contamination of the feedblock or die dueto degraded resin, (5) process control errors such as melt pressure,temperature and pumping rate variations, and (6) hydrodynamic flowinstabilities. The flow modeling should provide input to the feedblockand die designs in order to avoid conditions that could cause such flowinstabilities.

Overall thickness uniformity is affected by die design, casting wheelspeed fluctuations, system vibrations, die gap control, electrostaticpinning, and film stretching conditions. These variations can be eitherrandom or systematic. Systematic errors do not necessarily give aconstant (e.g., unchanging) color. For example, vibrations of the die orcasting wheel can cause a repeating spatial color variation with aperiodicity on the order of 0.5 to 50 cm. In certain applications suchas decorative film, where a periodic spatial color variation may bedesirable in the finished film, controlled periodic vibrations may beintentionally imparted to the casting wheel. However, where coloruniformity is desired and good thickness control is essential, thecasting wheel is fitted with a direct drive motor (e.g., no gearreduction). One example of such a motor is a D.C. brush servo motor,such as part number TT-10051A, available commercially from Kollmorgan.Higher speed motors with gear reduction can be used, but a high qualitysystem with proper electrical tuning and a smooth gearbox is essential.System vibrations, particularly of the die relative to the castingwheel, can be minimized by placing the casting station on concrete padson the ground floor of the casting installation. Other means ofdampening or isolation will be apparent to one skilled in the mechanicalarts.

The sources of vibrations can be identified with the help of a webthickness variation scanning device discussed earlier. If the period ofan oscillation can be identified from the output of such a device, asearch may be made for process elements, or even extraneous sources,which exhibit oscillatory behavior of identical period. These units canthen be made more rigid, vibration-damped, or vibration-isolated fromthe die and casting wheel by methods known in the art, or may simply beturned off or relocated if not essential to the process. Hence, avibration identified by periodicity as being due to the rotation of theextruder screw could be isolated, for example, by the use of a dampingmaterial between the extruder gate and the neck tube, while a vibrationidentified by periodicity as being due to a room fan could be removed byturning off or relocating the fan. In addition, a vibration of the dieor casting station which cannot be totally eliminated can be preventedfrom resulting in vibratory relative motion between the die and castingstation by mechanically linking the die to the casting station via someform of rigid superstructure. Many designs for such avibration-communicating mechanical linkage will be apparent.Furthermore, when strain hardening materials are employed in the film,stretching should be performed at sufficiently low temperatures toproduce a uniform stretch across the web, and the pinning wire should berigidly mounted.

Additional control over layer thickness and optical caliper is achievedthrough the use of a precision casting wheel drive mechanism having aconstant rotation speed. The casting wheel is designed and operated suchthat it is free of vibrations that would otherwise cause web thickness“chatter” and subsequent layer thickness variations in the down-webdirection. Applicants have found that those vibrations which produce arelative motion between the die and casting wheel result in effectivespeed variations in the casting wheel as it draws out the extrudatecoming from the die. These speed variations cause modulations in filmcaliper and optical layer thickness that are particularly pronounced inthe strain-hardening materials advantageously employed in making theoptical films of the present invention, resulting in color variationsacross the surface of the film. Accordingly, absent these controls atthe casting wheel, the normal vibrations encountered in the extrusionprocess are sufficient to noticeably diminish color uniformity in theoptical films of the present invention. The methods of the presentinvention have allowed the production, for the first time, of colorshifting films made from polymeric materials that have a high degree ofcolor uniformity at any particular viewing angle. Thus, films may bemade in accordance with the method of the present invention in which thedesired bandwidth of light transmitted or reflected at a particularangle of incidence varies by less than about 1 or 2 nm over an area ofat least 10 cm², and more preferably, at least 100 cm², and in which theabsolute bandedges of the spectral reflectance peaks vary in wavelengthby less than about +/−4 nm.

While thickness and/or color uniformity is important in manyapplications of the films of the present invention, in otherapplications, such as decorative films, color uniformity may be eitherunimportant or undesirable. In applications where color variations aredesirable, they may be intentionally imparted to the inventive opticalfilms by inducing thickness variations of a desired spatial frequencyacross or along a portion of the web at any point prior to quenching ofthe web in such a manner as to result in modulations in the thickness ofthe optical stack. While there are numerous ways of accomplishing thiseffect (e.g., by inducing vibrations in the casting wheel), suchmodulations may be conveniently imparted by inducing vibrations of adesired frequency (or frequencies) in the pinning wire. For example, byinducing a vibration on the pinning wire, the color of a polarizer filmwas periodically varied, in straight lines across the film, from aneutral gray transmission color to a red color. The red stripes were 6.0mm apart in the down-web direction. Calculated frequency of the pinningwire vibration was 21 Hz.

Local random color variations can also be achieved by extruding films ofthe present invention with small internal bubbles to produce attractivedecorative effects. Bubbles can be created by several methods includingnot drying the resin as sufficiently as one would normally do, or byslightly overheating a thermally sensitive resin such as PMMA to createa similar effect. The small bubbles formed locally distort themicrolayers and cause a local color change that can give the appearanceof depth in some instances.

Although the methods described above for inducing color variationsappear to teach a nonuniform film, the starting base film having uniformcolor with high stop band reflectivity and high color saturation,although locally disrupted by a given method, may be desirable incontrolling the average hue, color saturation, and brightness of such adecorative film. The local color variations taught here are morenoticeable when applied to a uniform color shifting film havingreflection bands with inherently high reflectivity and bandedges withhigh slopes.

As noted above, vibrations in the casting wheel cause the speed of thecasting wheel to fluctuate, resulting in variations of layer thicknessesin the film. The frequency (or frequencies) of the vibrations can bemodulated to impart repeating sequences or patterns of colors to theresulting film. Furthermore, these color variations can be accomplishedwithout destroying the color shifting characteristics typical of thefilms of the present invention, thereby allowing the production ofcolorful films (often spanning the entire visible spectrum) in which thecolors appear to shimmer or move as the angle of incidence is variedPeriodic color variations may also be imparted to the film by embossingit with a pattern. Due in part to the fact that the embossed portion isno longer coplanar with the rest of the film, it will exhibit adifferent color or colors than the rest of the film. Thus, strikingeffects have been produced by embossing the color shifting films of thepresent invention with, for example, a fishnet pattern (e.g., gold on ared background) or an emblem.

In certain instances, similar principles may be used to remove or tuneout periodic color variations in the film, thereby improving the coloruniformity of the film. Thus, where a source is found to impartvibrations of a given frequency or a given periodic frequency to theweb, vibrations of equal amplitude (but opposite phase) can be impartedto the web (e.g., through the casting wheel), resulting in destructiveinterference and effective removal of the source from the process.

Additional layers and coatings

As further steps in the process of making the high quality coextrudedpolymeric multilayer optical films of the present invention, variouslayers or coatings may be applied to at least a portion of one or bothsides of the multilayer optical stack to modify or enhance the physical,chemical, or optical characteristics of the film. These layers orcoatings may be integrated at the time of film formation, either bycoextrusion or in a separate coating or extrusion step, or they may beapplied to the finished optical film at a later time. Examples ofadditional layers or coatings are described in U.S. patent applicationSer. No. 08/910,660 filed Aug. 13, 1997 entitled “Multilayer PolymerFilm with Additional Coatings or Layers” which is incorporated herein byreference. A non-limiting listing of coatings or layers that may becombined with the coextruded multilayer film is described in more detailin the following examples.

A non-optical layer of material may be coextensively disposed on one orboth major surfaces of the film, i.e., the extruded optical stack. Thecomposition of the layer, also called a skin layer, may be chosen, forexample, to protect the integrity of the optical layers, to addmechanical or physical properties to the final film or to add opticalfunctionality to the final film. Suitable materials of choice mayinclude the material of one or more of the optical layers. Othermaterials with a melt viscosity similar to the extruded optical layersmay also be useful. It should also be noted that many of the mechanicaladvantages derived from skin layers can also be derived from ananalogous internal thick non-optical layer, e.g. a PBL.

A skin layer or layers may reduce the wide range of shear intensitiesthe extruded multilayer stack might experience within the extrusionprocess, particularly at the die. A high shear environment may causeundesirable deformations in the optical layers. A skin layer or layersmay also add physical strength to the resulting composite or reduceproblems during processing, such as, for example, reducing the tendencyfor the film to split during the orientation process. Skin layermaterials that remain amorphous can result in films having a highertoughness, while skin layer materials that are semicrystalline canresult in films having a higher tensile modulus. Other functionalcomponents such as antistatic additives, UV absorbers, dyes,antioxidants, and pigments, may be added to the skin layer, providedthey do not substantially interfere with the desired optical propertiesof the resulting product. Skin layers or coating may also be used to aidin post-extrusion processing; for example, by preventing sticking of thefilm to hot rollers or tenter clips.

Skin layers or coatings may also be added to impart desired barrierproperties to the resulting film or device. Thus, for example, barrierfilms or coatings may be added as skin layers, or as a component in skinlayers, to alter the transmissive properties of the film or devicetowards liquids, such as water or organic solvents, or gases, such asoxygen or carbon dioxide.

Skin layers or coatings may also be added to impart or improve abrasionresistance in the resulting article. Thus, for example, a skin layercomprising particles of silica embedded in a polymer matrix may be addedto an optical film produced in accordance with the invention to impartabrasion resistance to the film. Such a skin layer, however, should notunduly compromise the optical properties required for the application towhich the film is directed.

Skin layers or coatings may also be added to impart or improve punctureand/or tear resistance in the resulting article. Thus, for example, inembodiments in which the outer layer of the optical film contains coPEN,a skin layer of monolithic coPEN may be coextruded with the opticallayers to impart good tear resistance to the resulting film. Factors tobe considered in selecting a material for a tear resistant layer includepercent elongation to break, Young's modulus, tear strength, adhesion tointerior layers, percent transmittance and absorbance in anelectromagnetic bandwidth of interest, optical clarity or haze,refractive indices as a function of frequency, texture and roughness,melt thermal stability, molecular weight distribution, melt rheology andcoextrudability, miscibility and rate of inter-diffusion betweenmaterials in the skin and optical layers, viscoelastic response,relaxation and crystallization behavior under draw conditions, thermalstability at use temperatures, weatherability, ability to adhere tocoatings and permeability to various gases and solvents. Puncture ortear resistant skin layers may be applied during the manufacturingprocess or later coated onto or laminated to the optical film. Adheringthese layers to the optical film during the manufacturing process, suchas by a coextrusion process, provides the advantage that the opticalfilm is protected during the manufacturing process. In some embodiments,one or more puncture or tear resistant layers may be provided within theoptical film, either alone or in combination with a puncture or tearresistant skin layer.

The skin layers may be applied to one or two sides of the extrudedoptical stack at some point during the extrusion process, i.e., beforethe extruded and skin layer(s) exit the extrusion die. This may beaccomplished using conventional coextrusion technology, which mayinclude using a three-layer coextrusion die. Lamination of skin layer(s)to a previously formed multilayer film is also possible. Total skinlayer thicknesses may range from about 2% to about 50% of the totaloptical stack/skin layer thickness.

In some applications, additional layers may be coextruded or adhered onthe outside of the skin layers during manufacture of the optical films.Such additional layers may also be extruded or coated onto the opticalfilm in a separate coating operation, or may be laminated to the opticalfilm as a separate film, foil, or rigid or semi-rigid substrate such aspolyester (PET), acrylic (PMMA), polycarbonate, metal, or glass.

Many polymers are suitable for skin layers. Of the predominantlyamorphous polymers, suitable examples include copolyesters based on oneor more of terephthalic acid, 2,6-naphthalene dicarboxylic acid,isophthalic acid phthalic acid, or their alkyl ester counterparts, andalkylene diols, such as ethylene glycol. Examples of semicrystallinepolymers suitable for use in skin layers include 2,6-polyethylenenaphthalate, polyethylene terephthalate, and nylon materials. Skinlayers that may be used to increase the toughness of the optical filminclude high elongation polyesters such as ECDEL™ and PCTG 5445(available commercially from Eastman Chemical Co., Rochester, N.Y.) andpolycarbonates. Polyolefins, such as polypropylene and polyethylene, mayalso be used for this purpose, especially if they are made to adhere tothe optical film with a compatibilizer.

Various functional layers or coatings may be added to the optical filmsand devices of the present invention to alter or improve their physicalor chemical properties, particularly along the surface of the film ordevice. Such layers or coatings may include, for example, slip agents,low adhesion backside materials, conductive layers, antistatic coatingsor films, barrier layers, flame retardants, UV stabilizers, abrasionresistant materials, optical coatings, or substrates designed to improvethe mechanical integrity or strength of the film or device.

The optical films of the present invention may comprise a slip agentthat is incorporated into the film or added as a separate coating inorder to improve roll formation and convertibility of the film. In mostapplications, slip agents will be added to only one side of the film,ideally the side facing the rigid substrate in order to minimize haze.The films and optical devices of the present invention may be given goodslip properties by treating them with low friction coatings or slipagents, such as polymer beads coated onto the surface. Alternately, themorphology of the surfaces of these materials may be modified, asthrough manipulation of extrusion conditions, to impart a slipperysurface to the film; methods by which surface morphology may be somodified are described in U.S. Ser. No. 08/612,710.

The films and other optical devices made in accordance with theinvention may also be provided with one or more adhesives to laminatethe optical films and devices of the present invention to another film,surface, or substrate. Such adhesives include both optically clear anddiffuse adhesives, as well as pressure sensitive and non-pressuresensitive adhesives. Pressure sensitive adhesives are normally tacky atroom temperature and can be adhered to a surface by application of, atmost, light finger pressure, while non-pressure sensitive adhesivesinclude solvent, heat, or radiation activated adhesive systems. Examplesof adhesives useful in the present invention include those based ongeneral compositions of polyacrylate; polyvinyl ether; diene-containingrubbers such as natural rubber, polyisoprene, and polyisobutylene;polychloroprene; butyl rubber; butadiene-acrylonitrile polymers;thermoplastic elastomers; block copolymers such as styrene-isoprene andstyrene-isoprene-styrene block copolymers, ethylene-propylene-dienepolymers, and styrene-butadiene polymers; polyalphaolefins; amorphouspolyolefins; silicone; ethylene-containing copolymers such as ethylenevinyl acetate, ethylacrylate, and ethylmethacrylate; polyurethanes;polyamides; polyesters; epoxies; polyvinylpyrrolidone andvinylpyrrolidone copolymers; and mixtures of the above. Additionally,the adhesives can contain additives such as tackifiers, plasticizers,fillers, antioxidants, stabilizers, pigments, diffusing particles,curatives, and solvents. In some applications, as where the opticalfilms of the present invention are to be used as a component in adhesivetapes, it may be desirable to treat the films with low adhesion backsize(LAB) coatings or films such as those based on urethane, silicone orfluorocarbon chemistry. Films treated in this manner will exhibit properrelease properties towards pressure sensitive adhesives (PSAs), therebyenabling them to be treated with adhesive and wound into rolls Adhesivetapes, sheets, or die-cuts made in this manner can be used fordecorative purposes or in any application where a diffusely reflectiveor transmissive surface on the tape is desirable. When a laminatingadhesive is used to adhere an optical film of the present invention toanother surface, the adhesive composition and thickness are preferablyselected so as not to interfere with the optical properties of theoptical film. For example, when laminating additional layers to anoptical polarizer or mirror wherein a high degree of transmission isdesired, the laminating adhesive should be optically clear in thewavelength region that the polarizer or mirror is designed to betransparent in.

The films and optical devices of the present invention may also beprovided with one or more conductive layers. Such conductive layers maycomprise metals such as silver, gold, copper, aluminum, chromium,nickel, tin, and titanium, metal alloys such as silver alloys, stainlesssteel, and inconel, and semiconductor metal oxides such as doped andundoped tin oxides, zinc oxide, and indium tin oxide (ITO).

The films and optical devices of the present invention may also beprovided with antistatic coatings or films. Such coatings or filmsinclude, for example, V₂O₅ and salts of sulfonic acid polymers, carbonor other conductive metal layers.

The optical films and devices of the present invention may also beprovided with one or more barrier films or coatings that alter thetransmissive properties of the optical film towards certain liquids orgases. Thus, for example, the devices and films of the present inventionmay be provided with films or coatings that inhibit the transmission ofwater vapor, organic solvents, O₂, or CO₂ through the film. Barriercoatings will be particularly desirable in high humidity environments,where components of the film or device would be subject to distortiondue to moisture permeation.

The optical films and devices of the present invention may also betreated with flame retardants, particularly when used in environments,such as on airplanes, that are subject to strict fire codes. Suitableflame retardants include aluminum trihydrate, antimony trioxide,antimony pentoxide, and flame retarding organophosphate compounds.

The optical films and devices of the present invention may also beprovided with abrasion-resistant or hard coatings, which will frequentlybe applied as a skin layer. These include acrylic hardcoats such asAcryloid A-11 and Paraloid K-120N, available from Rohm & Haas,Philadelphia, Pa.; urethane acrylates, such as those described in U.S.Pat. No. 4,249,011 and those available from Sartomer Corp., Westchester,Pa.; and urethane hardcoats obtained from the reaction of an aliphaticpolyisocyanate (e.g., Desmodur N3300, available from Miles, Inc.,Pittsburgh, Pa.) with a polyester (e.g., Tone Polyol 0305, availablefrom Union Carbide, Houston, Tex.).

The optical films and devices of the present invention may further belaminated to rigid or semi-rigid substrates, such as, for example,glass, metal, acrylic, polyester, and other polymer backings to providestructural rigidity, weatherability, or easier handling. For example,the optical films of the present invention may be laminated to a thinacrylic or metal backing so that it can be stamped or otherwise formedand maintained in a desired shape. For some applications, such as whenthe optical film is applied to other breakable backings, an additionallayer comprising PET film or puncture-tear resistant film may be used.Additionally, for some applications such as in liquid crystal displays,the multilayer optical film may be combined with a light redirectingstructure as described in U.S. patent application Ser. No. 08/402,349filed Mar. 10, 1995. Such a light redirecting structure coated onto themultilayer optical film, laminated as a separated film, cast and curedon a multilayer optical film substrate, or embossed directly onto thesurface of the multilayer optical film.

The optical films and devices of the present invention may also beprovided with shatter resistant films and coatings. Films and coatingssuitable for this purpose are described, for example, in publications EP592284 and EP 591055, and are available commercially from 3M Company,St. Paul, Minn.

Various optical layers, materials, and devices may also be applied to,or used in conjunction with, the films and other optical devices of thepresent invention for specific applications. These include, but are notlimited to, magnetic or magneto-optic coatings or films; liquid crystalpanels, such as those used in display panels and privacy windows;photographic emulsions; fabrics; prismatic films, such as linear Fresnellenses; brightness enhancement films; holographic films or images;embossable films; anti-tamper films or coatings; IR transparent film forlow emissivity applications; release films or release coated paper; andpolarizers or mirrors. Multiple additional layers on one or both majorsurfaces of the optical film are contemplated, and can be anycombination of aforementioned coatings or films. For example, when anadhesive is applied to the optical film, the adhesive may contain awhite pigment such as titanium dioxide to increase the overallreflectivity, or it may be optically transparent to allow thereflectivity of the substrate to add to the reflectivity of the opticalfilm.

The films and other optical devices made in accordance with theinvention may include one or more anti-reflective layers or coatings,such as, for example, conventional vacuum coated dielectric metal oxideor metal/metal oxide optical films, silica sol gel coatings, and coatedor coextruded antireflective layers such as those derived from low indexfluoropolymers such as THV™, an extrudable fluoropolymer available from3M Company (St. Paul, Minn.). Such layers or coatings, which may or maynot be polarization sensitive, serve to increase transmission and toreduce reflective glare, and may be imparted to the films and opticaldevices of the present invention through appropriate surface treatment,such as coating or sputter etching. In some embodiments of the presentinvention, it is desired to maximize the transmission and/or minimizethe specular reflection for certain polarizations of light. In theseembodiments, the optical body may comprise two or more layers in whichat least one layer comprises an anti-reflection system in close contactwith a layer providing the continuous and disperse phases. Such ananti-reflection system acts to reduce the specular reflection of theincident light and to increase the amount of incident light that entersthe portion of the body comprising the continuous and disperse layers.Such a function can be accomplished by a variety of means well known inthe art. Examples are quarter wave anti-reflection layers, two or morelayer anti-reflective stack, graded index layers, and graded densitylayers. Such anti-reflection functions can also be used on thetransmitted light side of the body to increase transmitted light ifdesired.

The films and other optical devices made in accordance with theinvention may also be provided with a film or coating which impartsanti-fogging properties. In some cases, an anti-reflection layer asdescribed above will serve the dual purpose of imparting bothanti-reflection and anti-fogging properties to the film or device.Various anti-fogging agents are known to the art which are suitable foruse with the present invention. Typically, however, these materials willsubstances, such as fatty acid esters, which impart hydrophobicproperties to the film surface and which promote the formation of acontinuous, less opaque film of water. Several inventors have reportedcoatings that reduce the tendency for surfaces to “fog”. For example,U.S. Pat. No. 3,212,909 to Leigh discloses the use of ammonium soap,such as alkyl ammonium carboxylates in admixture with a surface activeagent which is a sulfated or sulfonated fatty material, to produce aanti-fogging composition. U.S. Pat. No. 3,075,228 to Elias discloses theuse of salts of sulfated alkyl aryloxypolyalkoxy alcohol, as well asalkylbenzene sulfonates, to produce an anti-fogging article useful incleaning and imparting anti-fogging properties to various surfaces. U.S.Pat. No. 3,819,522 to Zmoda, discloses the use of surfactantcombinations comprising derivatives of decyne diol as well as surfactantmixtures which include ethoxylated alkyl sulfates in an anti-foggingwindow cleaner surfactant mixture. Japanese Patent Kokai No. Hei6[1994]41,335 discloses a clouding and drip preventive compositioncomprising colloidal alumina, colloidal silica and an anionicsurfactant. U.S. Pat. No. 4,478,909 (Taniguchi et al) discloses a curedanti-fogging coating film which comprises polyvinyl alcohol, a finelydivided silica, and an organic silicon compound, the carbon/siliconweight ratio apparently being important to the film's reportedanti-fogging properties. Various surfactants, includefluorine-containing surfactants, may be used to improve the surfacesmoothness of the coating. Other anti-fog coatings incorporatingsurfactants are described in U.S. Pat. Nos. 2,803,552; 3,022,178; and3,897,356. World Patent No. PCT 96/18,691 (Scholtz et al) disclosesmeans by which coatings may impart both anti-fog and anti-reflectiveproperties.

The films and optical devices of the present invention may also beprotected from UV radiation through the use of UV stabilized films orcoatings. Suitable UV stabilized films and coatings include those whichincorporate benzotriazoles or hindered amine light stabilizers (HALS)such as Tinuvin™ 292, both of which are available commercially from CibaGeigy Corp., Hawthorne, N.Y. Other suitable UV stabilized films andcoatings include those which contain benzophenones or diphenylacrylates, available commercially from BASF Corp., Parsippany, N.J. Suchfilms or coatings will be particularly important when the optical filmsand devices of the present invention are used in outdoor applications orin luminaires where the source emits significant amount of light in theUV region of the spectrum.

The films and optical devices of the present invention may also includeantioxidants such as, for example, 4,4′-thiobis-(6-t-butyl-m-cresol),2,2′-methylenebis-(4-methyl-6-t-butyl-butylphenol),octadecyl-3,5-di-t-butyl-4-hydroxyhydrocinnamate,bis(2,4-di-t-butylphenyl) pentaerythritol diphosphite, Irganox™ 1093(1979)(((3,5-bis(1,1-dimethylethyl)-4-hydroxyphenyl)methyl)-dioctadecylester phosphonic acid), Irganox™ 1098(N,N′-1,6-hexanediylbis(3,5-bis(1,1-dimethyl)-4-hydroxy-benzenepropanamide),Naugaard™ 445 (aryl amine), Irganox™ 57 (alkylated diphenylamine),Irganox™ L 115 (sulfur containing bisphenol), Irganox™ LO 6 (alkylatedphenyl-delta-napthylamine), Ethanox 398 (flourophosphonite), and2,2′-ethylidenebis(4,6-di-t-butylphenyl)fluorophosnite. A group ofantioxidants that are especially preferred are sterically hinderedphenols, including butylated hydroxytoluene (BHT), Vitamin E(di-alpha-tocopherol), Irganox™ 1425WL (calciumbis-(O-ethyl(3,5-di-t-butyl-4-hydroxybenzyl))phosphonate), Irganox™ 1010(tetrakis(methylene(3,5,di-t-butyl-4-hydroxyhydrocinnamate))methane),Irganox™ 1076 (octadecyl 3,5-di-tert-butyl-4-hydroxyhydrocinnamate),Ethanox™ 702 (hindered bis phenolic), Ethanox 330 (high molecular weighthindered phenolic), and Ethanox™ 703 (hindered phenolic amine).

The films and optical devices of the present invention may also betreated with inks, dyes, or pigments to alter their appearance or tocustomize them for specific applications. Thus, for example, the filmsmay be treated with inks or other printed indicia such as those used todisplay product identification, advertisements, warnings, decoration, orother information. Various techniques can be used to print on the film,such as screen printing, letterpress, offset, flexographic printing,stipple printing, laser printing, and so forth, and various types of inkcan be used, including one and two component inks, oxidatively dryingand UV-drying inks, dissolved inks, dispersed inks, and 100% inksystems. The appearance of the optical film or other optical device mayalso be altered by coloring the device such as by laminating a dyed filmto the optical device, applying a pigmented coating to the surface ofthe optical device, or including a pigment in one or more of thematerials used to make the optical device. Both visible and near IR dyesand pigments are contemplated in the present invention, and include, forexample, optical brighteners such as dyes that absorb in the UV andfluoresce in the visible region of the color spectrum. Other additionallayers that may be added to alter the appearance of the optical filminclude, for example, opacifying (black) layers, diffusing layers,holographic images or holographic diffusers, and metal layers. Each ofthese may be applied directly to one or both surfaces of the opticalfilm, or may be a component of a second film or foil construction thatis laminated to the optical film. Alternately, some components such asopacifying or diffusing agents, or colored pigments, may be included inan adhesive layer which is used to laminate the optical film to anothersurface.

The films and devices of the present invention may also be provided withmetal coatings. Thus, for example, a metallic layer may be applieddirectly to the optical film by pyrolysis, powder coating, vapordeposition, cathode sputtering, ion plating, and the like. Metal foilsor rigid metal plates may also be laminated to the optical film, orseparate polymeric films or glass or plastic sheets may be firstmetallized using the aforementioned techniques and then laminated to theoptical films and devices of the present invention.

Dichroic dyes are a particularly useful additive for many of theapplications to which the films and optical devices of the presentinvention are directed, due to their ability to absorb light of aparticular polarization when they are molecularly aligned within thematerial. When used in a film or other optical body, the dichroic dyecauses the material to absorb one polarization of light more thananother. Suitable dichroic dyes for use in the present invention includeCongo Red (sodium diphenyl-bis-α-naphthylamine sulfonate), methyleneblue, stilbene dye (Color Index (CI)=620), and 1,1′-diethyl-2,2′-cyaninechloride (CI=374 (orange) or CI=518 (blue)). The properties of thesedyes, and methods of making them, are described in E. H. Land, ColloidChemistry (1946). These dyes have noticeable dichroism in polyvinylalcohol and a lesser dichroism in cellulose. A slight dichroism isobserved with Congo Red in PEN. Still other dichroic dyes, and methodsof making them, are discussed in the Kirk Othmer Encyclopedia ofChemical Technology, Vol. 8, pp. 652-661 (4th Ed. 1993), and in thereferences cited therein. Dychroic dyes in combination with certainpolymer systems exhibit the ability to polarize light to varyingdegrees. Polyvinyl alcohol and certain dichroic dyes may be used to makefilms with the ability to polarize light. Other polymers, such aspolyethylene terephthalate or polyamides, such as nylon-6, do notexhibit as strong an ability to polarize light when combined with adichroic dye. The polyvinyl alcohol and dichroic dye combination is saidto have a higher dichroism ratio than, for example, the same dye inother film forming polymer systems. A higher dichroism ratio indicates ahigher ability to polarize light. Combinations of a dichroic dye with amultilayer optical polarizer are described in U.S. patent applicationsSer. No. 08/402,042 entitled “Optical Polarizer” filed Mar. 10, 1995; inSer. No. 09/006,458 entitled “Dichroic Polarizing Film and OpticalPolarizers Containing the Film” filed Jan. 13, 1998; and in Ser. No.09/006,468 entitled “Optical Device with a Dichroic Polarizer and aMultilayer Optical Film” filed Jan. 13, 1998.

In addition to the films, coatings, and additives noted above, theoptical materials of the present invention may also comprise othermaterials or additives as are known to the art. Such materials includebinders, coatings, fillers, compatibilizers, surfactants, antimicrobialagents, foaming agents, reinforcers, heat stabilizers, impact modifiers,plasticizers, viscosity modifiers, and other such materials.

The films and other optical devices made in accordance with the presentinvention may be subjected to various treatments which modify thesurfaces of these materials, or any portion thereof, as by renderingthem more conducive to subsequent treatments such as coating, dying,metallizing, or lamination. This may be accomplished through treatmentwith primers, such as PVDC, PMMA, epoxies, and aziridines, or throughphysical priming treatments such as corona, flame, plasma, flash lamp,sputter-etching, e-beam treatments, or amorphizing the surface layer toremove crystallinity, such as with a hot can.

For some applications, it may also be desirable to provide the films andother optical devices of the present invention one or more layers havingcontinuous and disperse phases in which the interface between the twophases will be sufficiently weak to result in voiding when the film isoriented. The average dimensions of the voids may be controlled throughcareful manipulation of processing parameters and stretch ratios, orthrough selective use of compatibilizers. The voids may be back-filledin the finished product with a liquid, gas, or solid. Voiding may beused in conjunction with the specular optics of the optical stack toproduce desirable optical properties in the resulting film.

Converting

Various lubricants may also be used during the processing (e.g.,extrusion) of the films. Suitable lubricants for use in the presentinvention include calcium stearate, zinc stearate, copper stearate,cobalt stearate, molybdenum neodocanoate, and ruthenium (III)acetylacetonate. In addition, the film may undergo subsequent processingsteps such as converting, wherein the film may be slit into rolls orfinished sheets for a particular use, or the film may be slit orconverted into strips, fibers, or flakes such as are used for glitter.Depending on the end-use application, additional coatings or layers asdescribed above may be added either prior to or after a convertingoperation.

The multilayer optical films made according to the present invention maybe converted into glitter in any of a variety of desired shapes andsizes (including copyrightable material or a trademark, e.g. movie or TVcharacters), including a registerable trademark or registered copyrightas defined under the laws of the countries, territories, etc. of theworld (including those of the United States). The periphery of theglitter may be, for example, a regular, predetermined shape (e.g.,circles, squares, rectangles, diamonds, stars, or alphanumerics, otherpolygons (e.g., hexagons)), or an irregular random shape and mixtures ofat least two different shapes and/or sizes. The size and shape of theglitter is typically chosen to optimize the appearance of the glitter orto suit a particular end use application. Typically, at least a portionof the glitter has particle sizes (i.e., maximum particle dimension)less than about 10 mm; more typically less than about 3 mm. In anotheraspect, at least a portion of the glitter typically has particle sizesranging from about 50 micrometers to about 3 mm; preferably from about100 micrometers to about 3 mm. Conversion of the film into regular,predetermined shapes is typically done using precision cuttingtechniques (e.g., rotary die cutting). Conversion services arecommercially available, for example, from Glitterex Corporation,Belleville, N.J.

The thickness of the multilayer optical film comprising glitter istypically less than about 125 micrometers, more typically less than 75micrometer, and preferably less than 50 micrometers, and thickness maygo down to 15 micrometers for applications such as automobile paint).Multi-layer films suitable for use in making glitter according to thepresent invention preferably have sufficient inter-layer adhesion toprevent delamination during the conversion process. The thickness of thefilm (in the z direction) from which glitter according to the presentinvention is preferably about 3 to about 25% of the smallest glitterparticle dimension (i.e., measured in the respective x and ydirections). Preferably, the glitter is sufficiently thick to remainflat in application, but not so thick as to create substantial edgeeffects (i.e., distortions on cut edges of the glitter particles thatextend into a substantial portion of the film thickness).

The glitter may be incorporated into a matrix material material (e.g., across-linked polymeric material) in one or more subsequent steps. In oneembodiment the glitter is dispersed (e.g., uniformly or non-uniformly)within a translucent (including transparent) matrix material such thatat least a portion of the glitter is observable by a viewer of thecomposite material comprising the matrix material and the glitter. Thematrix material need not be translucent (i.e., can be opaque) providedthat glitter is at the outer surface of the matrix material such that atleast a portion of the glitter is observable by a viewer of the article.The glitter made according to the present invention may also provide anarticle or composition comprising a substrate, a matrix disposed on thesubstrate, and a plurality of glitter disposed in the matrix.

Techniques for incorporating glitter made according to the presentinvention into the matrix material include those known in the art forincorporating conventional glitters into matrix materials. For example,glitter can be dispersed in a liquid, for example, by mixing orotherwise agitating the liquid with glitter therein. Dispersion of theglitter in the liquid may be aided, for example, with the use ofdispersion aids. In some cases, a liquid having glitter dispersedtherein is a precursor for a composite article derived therefrom. Forexample, glitter can be dispersed in a curable polymeric materialwherein the glitter containing polymeric material is placed in a moldhaving the shape of the desired final article, followed by the curing ofthe polymeric material.

Articles comprising glitter-containing matrix materials may be made byany of a variety of techniques including cast molding, injection molding(particularly useful, for example, to make three-dimensional articles);extrusion (particularly useful, for example, to make films, sheetmaterials, fibers and filaments, cylindrical tubes, and cylindricalshells (i.e., pipe). Sheet or film materials may comprise a single layeror a plurality of layers (i.e., a multiple-layered construction).Multiple layer constructions may have the glitter in one or more of thelayers, and may optionally contain different shapes, sizes, andconcentrations of glitter in different layers. Further, for example,glitter made according to the present invention may be incorporatedinto, or mixed with, polymer pellets suitable for injection molding.Other examples of processes for incorporating glitter according to thepresent invention into a matrix material of a finished article includevacuum molding, blow molding, rotomolding, thermoforming, extruding,compression molding, and calendering.

Articles incorporating glitter made according to the present inventionmay, for example, have the glitter uniformly or non-uniformly (includingrandomly) dispersed therein and/or thereon, as well have some areas withthe glitter uniformly or non-uniformly dispersed therein and/or thereon,and other areas wherein it is non-uniformly or uniformly, respectively,dispersed therein and/or thereon. Further, the glitter may be presentsuch that there are concentration gradients of glitter.

The present process may include the step of orientation of the glitterin the matrix material. The glitter particles may, for example, berandom with respect to one another, or have substantially theorientation relative to one another or relative to a surface of thematrix material. Alignment or orientation of the glitter within thematrix material may be provided, for example, by high shear processing(e.g., extrusion or injection molding) of glitter-containing matrixmaterial which results in orientation or alignment of the glitter alongthe flow direction of the matrix material. Other techniques fororientating the glitter within a matrix material may be apparent tothose skilled in the art after reviewing the disclosure of the presentinvention.

The glitter may also be randomly or uniformly distributed over thesurface of an article, and can be random in some areas of the surfaceand uniform in others. Further, for example, the glitter can be randomlyor uniformly (e.g., uniformly spaced) oriented with respect to thesurface, and can be randomly oriented in some areas and uniformlyoriented in others. The glitter can be patterned to provide, or be apart of, copyrightable material or a trademark (e.g. movie or TVcharacters), including a registered or registrable trademark under anyof the laws of the countries, territories, etc. of the world.Optionally, a coating (e.g., a clear coating) may be applied over atleast a portion of the glitter to provide additional bonding to thesubstrate, to provide protection to the glitter, or to provide a morevisually appealing effect.

Turning again to liquids having glitter according to the presentinvention therein, such dispersions, or dispersible combinations may besolvent-borne (i.e., dissolved in an organic solvent), water-borne(i.e., dissolved or dispersed in water), single component, ormulti-component. When the dispersions, or dispersible combinations areto be used to provide a coating on a surface, the liquid may preferablybe a film-forming material.

Examples of liquid mediums, although the compatibility(e.g., chemicalcompatibility), and hence the suitability of a particular liquid willdepend, for example on the composition of the glitter, as well as othercomponents of the dispersions, or dispersible combinations, includewater, organic liquids (e.g., alcohols, ketones (for a short period oftime)), and mixtures thereof. It is noted that some matrix materials maysometimes be liquids, and other times a solid. For example, at roomtemperature, typical hot melt adhesive materials are solids, whereaswhen heated to their respective melting points, they are liquids.Further, for example, liquid glue, prior to curing and/or drying is aliquid, but after curing and/or drying, is a solid.

The dispersions, or dispersible combinations, may be, for example,dryable, curable, or the like to form yet another matrix (e.g., a paintmay be dried or cured to provided a solid or hardened form). Thedispersions, or dispersible combinations, may include additives (e.g.,antimicrobials, antistats, blowing agents, colorants or pigments (e.g.,to tint, or otherwise impart or alter the color of, the matrixmaterial), curatives, thinners, fillers, flame retardants, impactmodifiers, initiators, lubricants, plasticisers, slip agents,stabilizers, and coalescing aids, thickening aids, dispersion aids,defoamers, and biocides) which provide, for example, a desirable featureor property in the desired final composite (comprising the glitter),and/or aid in the processing step(s) to make the desired final composite(comprising the glitter).

In one aspect, the dispersion, or dispersible combination includesbinder precursor material (i.e., a material that is convertable from aliquid (i.e., a flowable form; e.g., polymers dissolved in a solvent,polymer precursors dissolved in a solvent, polymer emulsions, andcurable liquids) into a solidified or hardened form. Processes toconvert a liquid binder precursor material to a solidified or hardenedbinder material include evaporation of a solvent, curing (i.e.,hardening via chemical reaction), and combinations thereof.

Additional examples of binder precursors and binders for thedispersions, or dispersible combinations, containing glitter accordingto the present invention include vinyl polymers, vinyl-acrylic polymers,acrylic polymers, vinyl-chloride acrylic polymers, styrene/butadienecopolymers, styrene/acrylate copolymers, vinyl acetate/ethylenecopolymers, animoalkyl resin, thermosetting acrylic resins,nitrocellulose resins, modified acrylic lacquer, straight chain acryliclacquer, polyurethane resin, acrylic enamel resin, silylgroup-containing vinyl resin, and combinations thereof.

Examples of dispersions or dispersible combinations, that can containglitter according to the present invention include fingernail polish,paint (including paint for automotive and marine applications, indoorand outdoor house paint, art and crafts paint, hobby paints (e.g., toymodel paints), and finger paints). Such dispersions or dispersiblecombinations, are typically applied to a surface to provide a coatingwhich is subsequently dried, cured, or the like to provide a hardened ornon-wet surface coating.

The size, shape, thickness, and amount of glitter used in a particularapplication, including applications described herein, may depend on anumber of factors, including the desired effect to be achieved, cost,inherent limitations of the application (e.g., if the glitter is in abinder material, the amount of glitter should not exceed the loadingcapacity of the binder matrix, unless it is desired for excess glitterto easily fall out), and for liquid matrices, the viscosity of thedispersions, or other physical properties or performance characteristicsof a matrix having the glitter therein. Glitter made according to thepresent invention may also be applied to a surface by first applying abinder or adhesive material, then applying the glitter, followed bydrying, curing, solidification, or the like of the binder or adhesivematerial. Examples of substrate for adhering the glitter to includetoys, fabrics, sheet materials (e.g., paper, cardboard, and films),ornaments, plastics, wood, and metal. Adhering glitter to the surface ofa substrate can, for example, provide a decorative effect.

The glitter may be adhered to the surface using any suitable form ofattachment, such as glue, pressure sensitive adhesive, hot-meltadhesive, and stitching. When adhered with adhesive materials, theglitter can, for example, be placed onto, or broadcasted over, thesurface of the adhesive-coated substrate. Placement of the glitterrelative to the substrate may be provided in any of a variety of desiredpatterns and/or orientations. For example, the glitter can be randomlyor uniformly over the surface, and can be random in some areas of thesurface and uniform in others. Further, for example, the glitter can berandomly or uniformly (e.g., uniformly spaced) oriented with respect tothe surface, and can be randomly oriented in some areas and uniformlyoriented in others. The glitter can be patterned to provide, or be apart of, copyrightable material or a trademark (e.g. movie or TVcharacters), including a registered or registerable trademark under anyof the laws of the countries, territories, etc. of the world.Optionally, a coating (e.g., a clear coating) may be applied over atleast a portion of the glitter to provide additional bonding to thesubstrate, to provide protection to the glitter, or to provide a morevisually appealing effect.

Additional processing steps such as are commonly known in the filmprocessing art may also be used in the processing of coextrudedpolymeric multilayer optical films of the present invention. The presentinvention should not be considered limited to the particular examplesdescribed above, but rather should be understood to cover all aspects ofthe invention as fairly set out in the attached claims. Variousmodifications, equivalent processes, as well as numerous structures towhich the present invention may be applicable will be readily apparentto those of skill in the art to which the present invention is directedupon review of the present specification. The claims are intended tocover such modifications and devices.

What is claimed is:
 1. A method for making a multilayered optical film,comprising: (a) providing at least a first and a second stream of resin;(b) providing a feeder tube plate having a first plurality of conduits,and having a second plurality of conduits interleaved with the firstplurality of conduits; (c) feeding the first stream into the firstplurality of conduits and the second stream into the second plurality ofconduits; (d) providing a slot die for each of the first and secondplurality of conduits such that the first and second streams are dividedinto a plurality of interleaved layers to yield a composite stream; (e)passing the composite stream through an extrusion die to form amultilayer web in which each layer is generally parallel to the majorsurface of adjacent layers; and (f) heating the first and second streamsof resin with at least one axial rod heater that extends along at leastone of the first and second plurality of conduits.
 2. The method ofclaim 1, further comprising: (g) positioning the at least one axial rodheater proximate the first and second plurality of conduits.
 3. Themethod of claim 1, further comprising: (g) positioning the at least oneaxial rod heater between the first and second plurality of conduits. 4.The method of claim 1, further comprising: (g) varying temperature ofthe at least one axial rod heater in zones along a length of the axialrod heater.
 5. The method of claim 1, wherein the heating step comprisesheating with a plurality of axial rod heaters.
 6. The method of claim 5,wherein the healing step comprises heating with a first axial rod heaterdisposed proximate the first plurality of conduits and heating with asecond axial rod heater disposed proximate the second plurality ofconduits.
 7. The method of claim 1, further comprising after step (d):(g) passing the composite stream into a multiplier where the compositestream is divided into a plurality of substreams, the multiplierexpanding at least one of the substreams in a direction transverse toits direction of flow; and (h) recombining the substreams to increasethe number of layers in the composite stream.
 8. The method of claim 1,further comprising: (g) casting the multilayer web onto a casting rollto form a cast multilayer film.
 9. The method of claim 8, furthercomprising: (h) quenching said multilayer web during the casting step.